vm: Fix 'vm --full-boot' to produce a sufficient disk image.

* gnu/system/vm.scm (system-qemu-image/shared-store): Add
  #:disk-image-size and #:full-boot? parameters and honor them.  Pass
  '#:copy-inputs? full-boot?', and change #:inputs argument.
* guix/scripts/system.scm (system-derivation-for-action): Pass
  #:disk-image-size to 'system-qemu-image/shared-store'.
* doc/guix.texi (Invoking guix system): Mention use of '--image-size' in
  conjunction with '--full-boot'.

doc/guix.texi

@@ -4151,9 +4151,15 @@ Build a virtual machine that contain the operating system declared in
 
 The VM shares its store with the host system.
 
-On GNU/Linux, the default is to boot directly to the kernel.  The
-@code{--full-boot} option forces a complete boot sequence, starting with
-the bootloader.
+On GNU/Linux, the default is to boot directly to the kernel; this has
+the advantage of requiring only a very tiny root disk image since the
+host's store can then be mounted.
+
+The @code{--full-boot} option forces a complete boot sequence, starting
+with the bootloader.  This requires more disk space since a root image
+containing at least the kernel, initrd, and bootloader data files must
+be created.  The @code{--image-size} option can be used to specify the
+image's size.
 
 @item vm-image
 @itemx disk-image

gnu/system/vm.scm

@@ -376,20 +376,31 @@ environment with the store shared with the host."
 
 (define* (system-qemu-image/shared-store
           os
-          #:key (disk-image-size (* 15 (expt 2 20))))
+          #:key
+          full-boot?
+          (disk-image-size (* (if full-boot? 500 15) (expt 2 20))))
   "Return a derivation that builds a QEMU image of OS that shares its store
-with the host."
-  (mlet* %store-monad
-      ((os-drv      (operating-system-derivation os))
+with the host.
+
+When FULL-BOOT? is true, return an image that does a complete boot sequence,
+bootloaded included; thus, make a disk image that contains everything the
+bootloader refers to: OS kernel, initrd, bootloader data, etc."
+  (mlet* %store-monad ((os-drv   (operating-system-derivation os))
                        (grub.cfg (operating-system-grub.cfg os)))
+    ;; XXX: When FULL-BOOT? is true, we end up creating an image that contains
+    ;; GRUB.CFG and all its dependencies, including the output of OS-DRV.
+    ;; This is more than needed (we only need the kernel, initrd, GRUB for its
+    ;; font, and the background image), but it's hard to filter that.
     (qemu-image #:os-derivation os-drv
                 #:grub-configuration grub.cfg
                 #:disk-image-size disk-image-size
-                #:inputs `(("system" ,os-drv))
+                #:inputs (if full-boot?
+                             `(("grub.cfg" ,grub.cfg))
+                             '())
 
                 ;; XXX: Passing #t here is too slow, so let it off by default.
                 #:register-closures? #f
-                #:copy-inputs? #f)))
+                #:copy-inputs? full-boot?)))
 
 (define* (common-qemu-options image)
   "Return the a string-value gexp with the common QEMU options to boot IMAGE."
@@ -406,15 +417,23 @@ with the host."
                                                 #:key
                                                 (qemu qemu)
                                                 (graphic? #t)
-                                                full-boot?)
+                                                full-boot?
+                                                (disk-image-size
+                                                 (* (if full-boot? 500 15)
+                                                    (expt 2 20))))
   "Return a derivation that builds a script to run a virtual machine image of
-OS that shares its store with the host.  When FULL-BOOT? is true, the returned
-script runs everything starting from the bootloader; otherwise it directly
-starts the operating system kernel."
-  (mlet* %store-monad
-      ((os ->  (virtualized-operating-system os))
+OS that shares its store with the host.
+
+When FULL-BOOT? is true, the returned script runs everything starting from the
+bootloader; otherwise it directly starts the operating system kernel.  The
+DISK-IMAGE-SIZE parameter specifies the size in bytes of the root disk image;
+it is mostly useful when FULL-BOOT?  is true."
+  (mlet* %store-monad ((os ->  (virtualized-operating-system os))
                        (os-drv (operating-system-derivation os))
-       (image  (system-qemu-image/shared-store os)))
+                       (image  (system-qemu-image/shared-store
+                                os
+                                #:full-boot? full-boot?
+                                #:disk-image-size disk-image-size)))
     (define builder
       #~(call-with-output-file #$output
           (lambda (port)

guix/scripts/system.scm

@@ -258,7 +258,9 @@ it atomically, and then run OS's activation script."
     ((vm-image)
      (system-qemu-image os #:disk-image-size image-size))
     ((vm)
-     (system-qemu-image/shared-store-script os #:full-boot? full-boot?))
+     (system-qemu-image/shared-store-script os
+                                            #:full-boot? full-boot?
+                                            #:disk-image-size image-size))
     ((disk-image)
      (system-disk-image os #:disk-image-size image-size))))