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/api/viewsets.py

@@ -302,7 +302,6 @@ class CreateInstanceViewSet(viewsets.ViewSet):
                     volumes=volume_list,
                     networks=serializer.validated_data["networks"],
                     nwfilter=serializer.validated_data["nwfilter"],
-                    graphics=serializer.validated_data["graphics"],
                     virtio=serializer.validated_data["virtio"],
                     listener_addr=serializer.validated_data["listener_addr"],
                     video=serializer.validated_data["video"],