feat(console): VNC is the only console type

SPICE was removed earlier; its leftovers and the console type choice go
too:
- the console settings form, update_console and the backend no longer
  offer or change the type; an invalid console form now shows its
  errors instead of being ignored;
- the create form, the API serializer and create_instance no longer take
  a graphics type, new VMs get VNC; get_graphics_types and the graphics
  entries of the domain capabilities are gone;
- migration appsettings.0013 deletes QEMU_CONSOLE_DEFAULT_TYPE;
- a VM defined elsewhere with another graphics type gets "Console type
  ... is not supported".

The locale catalogs still reference the deleted SPICE templates in
source comments; regenerating them is left to the translation work.

instances/tests.py

@@ -780,7 +780,7 @@ class InstancesTestCase(TestCase):
     def test_console(self):
         response = self.client.post(
             reverse("instances:update_console", args=[self.instance.id]),
-            {"type": "spice", "listen_on": "0.0.0.0", "password": "", "keymap": "auto"},
+            {"listen_on": "0.0.0.0", "password": "", "keymap": "auto"},
             HTTP_REFERER=reverse("index"),
         )
         self.assertEqual(response.status_code, 302)