vm: Add #:name parameter for 'system-disk-image'.

* gnu/system/vm.scm (system-disk-image): Add #:name parameter; pass it
  to 'qemu-image'.

gnu/system/vm.scm

@@ -261,6 +261,7 @@ (define* (qemu-image #:key
 
 (define* (system-disk-image os
                             #:key
+                            (name "disk-image")
                             (file-system-type "ext4")
                             (disk-image-size (* 900 (expt 2 20)))
                             (volatile? #t))
@@ -296,7 +297,8 @@ (define file-systems-to-keep
 
     (mlet* %store-monad ((os-drv   (operating-system-derivation os))
                          (grub.cfg (operating-system-grub.cfg os)))
-      (qemu-image #:grub-configuration grub.cfg
+      (qemu-image #:name name
+                  #:grub-configuration grub.cfg
                   #:disk-image-size disk-image-size
                   #:disk-image-format "raw"
                   #:file-system-type file-system-type