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.

appsettings/migrations/0013_remove_qemu_console_default_type.py

@@ -0,0 +1,25 @@
+from django.db import migrations
+
+
+def remove_console_type(apps, schema_editor):
+    """VNC is the only console type; the setting has nothing left to choose."""
+    AppSettings = apps.get_model("appsettings", "AppSettings")
+    AppSettings.objects.using(schema_editor.connection.alias).filter(key="QEMU_CONSOLE_DEFAULT_TYPE").delete()
+
+
+def restore_console_type(apps, schema_editor):
+    AppSettings = apps.get_model("appsettings", "AppSettings")
+    AppSettings.objects.using(schema_editor.connection.alias).get_or_create(
+        key="QEMU_CONSOLE_DEFAULT_TYPE",
+        defaults={"name": "VM Console Type", "value": "vnc", "choices": "vnc", "description": "Default console type"},
+    )
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("appsettings", "0012_remove_spice_qemu_console_type"),
+    ]
+
+    operations = [
+        migrations.RunPython(remove_console_type, restore_console_type),
+    ]