vm: Use a para-virtualized disk when creating an image.

* guix/build/vm.scm (load-in-linux-vm): When MAKE-DISK-IMAGE?, use
  '-drive ...,if=virtio' for better performance.
* gnu/system/vm.scm (qemu-image): Use /dev/vda instead of /dev/sda.

gnu/system/vm.scm

@@ -236,7 +236,7 @@ the image."
           (let ((graphs '#$(match inputs
                              (((names . _) ...)
                               names))))
-            (initialize-hard-disk "/dev/sda"
+            (initialize-hard-disk "/dev/vda"
                                   #:grub.cfg #$grub-configuration
                                   #:closures graphs
                                   #:copy-closures? #$copy-inputs?

guix/build/vm.scm

@@ -96,7 +96,8 @@ the #:references-graphs parameter of 'derivation'."
                   "-append" (string-append "console=ttyS0 --load="
                                            builder)
                   (if make-disk-image?
-                      `("-hda" ,image-file)
+                      `("-drive" ,(string-append "file=" image-file
+                                                 ",if=virtio"))
                       '())))
     (error "qemu failed" qemu))