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>