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/templates/instance.html

@@ -266,13 +266,6 @@
             $("#console_select_keymap option[value='" + keymap + "']").prop('selected', true);
         }
     });
-    $(document).ready(function () {
-        // set current console type or fall back to default
-        var console_type = "{{ instance.console_type }}";
-        if (console_type != '') {
-            $("#console_select_type option[value='" + console_type + "']").prop('selected', true);
-        }
-    });
     $(document).ready(function () {
         // set current console listen address or fall back to default
         var console_listener_address = "{{ instance.console_listener_address }}";