Some convinience fluff:  Add support for '-vga virtio', also add
virtio-vga to the list of vga cards so '-device virtio-vga' will
turn off the default vga.

Written by Dave Airlie and Gerd Hoffmann.
Signed-off-by: NDave Airlie <airlied@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>