system: vm: Include the cirrus driver in the initrd.

This is Ludo's suggestion at <https://issues.guix.gnu.org/36069#26>,
with an added guard against duplicates and a tweaked comment.

* gnu/system/vm.scm (virtualized-operating-system): Add "cirrus" to the
list of INITRD-MODULES.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>
Change-Id: I1a86286586eee0b8c39706544778c7911a86da33

gnu/system/vm.scm

@@ -168,6 +168,12 @@ environment with the store shared with the host.  MAPPINGS is a list of
                      file-systems
                      #:volatile-root? volatile?
                      rest)))
+    ;; The (QEMU-only) "cirrus" graphics driver is still expected by some
+    ;; VPS with old QEMU versions.  See <https://bugs.gnu.org/36069>.
+    (initrd-modules (let ((modules (operating-system-initrd-modules os)))
+                      (if (member "cirrus" modules)
+                          modules
+                          (cons "cirrus" modules))))
 
     ;; Disable swap.
     (swap-devices '())