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/forms.py

@@ -1,6 +1,5 @@
 import re
 
-from appsettings.models import AppSettings
 from django import forms
 from django.utils.translation import gettext_lazy as _
 from webvirtcloud.settings import QEMU_CONSOLE_LISTENER_ADDRESSES, QEMU_KEYMAPS
@@ -15,7 +14,6 @@ class FlavorForm(forms.ModelForm):
 
 
 class ConsoleForm(forms.Form):
-    type = forms.ChoiceField(label=_("Type"))
     listen_on = forms.ChoiceField(label=_("Listen on"))
     generate_password = forms.BooleanField(label=_("Generate password"), required=False)
     clear_password = forms.BooleanField(label=_("Clear password"), required=False)
@@ -29,12 +27,7 @@ class ConsoleForm(forms.Form):
 
     def __init__(self, *args, **kwargs):
         super(ConsoleForm, self).__init__(*args, **kwargs)
-        type_choices = (
-            (c, c)
-            for c in AppSettings.objects.get(key="QEMU_CONSOLE_DEFAULT_TYPE").choices_as_list()
-        )
         keymap_choices = [("auto", _("Auto"))] + list((c, c) for c in QEMU_KEYMAPS)
-        self.fields["type"] = forms.ChoiceField(label=_("Type"), choices=type_choices)
         self.fields["listen_on"] = forms.ChoiceField(
             label=_("Listen on"),
             choices=QEMU_CONSOLE_LISTENER_ADDRESSES