services: hurd-vm: Check whether /dev/kvm exists at run time.

This change allows a childhurd to run within Guix System in a VM.

* gnu/services/virtualization.scm (hurd-vm-shepherd-service)[vm-command]:
Stage the 'file-exists?' call.

gnu/services/virtualization.scm

@@ -937,13 +937,14 @@ is added to the OS specified in CONFIG."
         (provisions  '(hurd-vm childhurd)))
 
     (define vm-command
-      #~(list
-         (string-append #$qemu "/bin/qemu-system-i386")
-         #$@(if (file-exists? "/dev/kvm") '("--enable-kvm") '())
+      #~(append (list #$(file-append qemu "/bin/qemu-system-i386")
                       "-m" (number->string #$memory-size)
                       #$@net-options
                       #$@options
-         "--hda" #+image))
+                      "--hda" #+image)
+                (if (file-exists? "/dev/kvm")
+                    '("--enable-kvm")
+                    '())))
 
     (list
      (shepherd-service