Use virtio graphics for server_cockpit_updates test
With qxl it hits a weird bug where running startx a second time
after snapshot restore tends to crash. This is affecting updates
so let's force it to virtio. Hopefully the kernel build with
virtio fix will succeed so I can just revert all this messing
around soon.
Signed-off-by: Adam Williamson <awilliam@redhat.com>