vm: Use grub-hybrid's grub-mkrescue.

* gnu/system/vm.scm (system-disk-image): Use grub-hybrid's grub-mkrescue.
* gnu/bootlader/grub.scm (grub-mkrescue-bootloader): New variable.

gnu/bootloader/grub.scm

@@ -55,6 +55,7 @@
 
             grub-bootloader
             grub-efi-bootloader
+            grub-mkrescue-bootloader
 
             grub-configuration))
 
@@ -410,6 +411,11 @@ submenu \"GNU system, old configurations...\" {~%")
    (name 'grub-efi)
    (package grub-efi)))
 
+(define* grub-mkrescue-bootloader
+  (bootloader
+   (inherit grub-efi-bootloader)
+   (package grub-hybrid)))
+
 
 ;;;
 ;;; Compatibility macros.

gnu/system/vm.scm

@@ -49,6 +49,7 @@
   #:use-module (gnu packages admin)
 
   #:use-module (gnu bootloader)
+  #:use-module ((gnu bootloader grub) #:select (grub-mkrescue-bootloader))
   #:use-module (gnu system shadow)
   #:use-module (gnu system pam)
   #:use-module (gnu system linux-initrd)
@@ -369,6 +370,12 @@ to USB sticks meant to be read-only."
                                #:volatile-root? #t
                                rest)))
 
+              (bootloader (if (string=? "iso9660" file-system-type)
+                              (bootloader-configuration
+                                (inherit (operating-system-bootloader os))
+                                (bootloader grub-mkrescue-bootloader))
+                              (operating-system-bootloader os)))
+
               ;; Force our own root file system.
               (file-systems (cons (file-system
                                     (mount-point "/")