vm: The 'run-vm' scripts now passes its arguments to QEMU.

* gnu/system/vm.scm (system-qemu-image/shared-store-script): Add "$@" at
  the end of the script.
  (common-qemu-options): Remove trailing newline.
* doc/guix.texi (Invoking guix system): Document it.

doc/guix.texi

@@ -4148,6 +4148,7 @@ This command also installs GRUB on the device specified in
 @cindex virtual machine
 Build a virtual machine that contain the operating system declared in
 @var{file}, and return a script to run that virtual machine (VM).
+Arguments given to the script are passed as is to QEMU.
 
 The VM shares its store with the host system.
 

gnu/system/vm.scm

@@ -411,7 +411,7 @@ (define* (common-qemu-options image)
   -serial stdio \
   -drive file=" #$image
   ",if=virtio,cache=writeback,werror=report,readonly \
-  -m 256\n"))
+  -m 256"))
 
 (define* (system-qemu-image/shared-store-script os
                                                 #:key
@@ -447,7 +447,8 @@ (define builder
             -initrd " #$os-drv "/initrd \
             -append \"" #$(if graphic? "" "console=ttyS0 ")
             "--system=" #$os-drv " --load=" #$os-drv "/boot --root=/dev/vda1\" "))
-#$(common-qemu-options image))
+#$(common-qemu-options image)
+" \"$@\"\n")
              port)
             (chmod port #o555))))