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/models.py

@@ -273,9 +273,6 @@ class CreateInstance(models.Model):
     console_pass = models.CharField(max_length=64, blank=True)
     add_cdrom = models.CharField(max_length=16)
     add_input = models.CharField(max_length=16)
-    graphics = models.CharField(
-        max_length=16, error_messages={"required": _("Please select a graphics type")}
-    )
     video = models.CharField(
         max_length=16, error_messages={"required": _("Please select a video driver")}
     )