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>

templates.fif.json

@@ -2161,6 +2161,7 @@
                 "+START_AFTER_TEST": "server_cockpit_default",
                 "BOOTFROM": "c",
                 "POSTINSTALL": "server_cockpit_updates server_cockpit_autoupdate",
+                "QEMUVGA": "virtio",
                 "ROOT_PASSWORD": "weakpassword",
                 "USER_LOGIN": "false"
             }