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/test_update_console.py

@@ -0,0 +1,27 @@
+from unittest.mock import patch
+
+from computes.models import Compute
+from django.contrib.auth import get_user_model
+from django.test import TestCase
+from django.urls import reverse
+from instances.models import Instance
+
+
+class UpdateConsoleTestCase(TestCase):
+    def setUp(self):
+        admin = get_user_model().objects.create_superuser("console_admin", "c@example.com", "pw")
+        self.client.force_login(admin)
+        compute = Compute.objects.create(name="console-compute", hostname="127.0.0.1", login="", password="", type=4)
+        self.instance = Instance.objects.create(
+            compute=compute, name="console-vm", uuid="11111111-2222-3333-4444-555555555555"
+        )
+
+    def post(self, data):
+        with patch("instances.models.wvmInstance") as wvm:
+            self.client.post(reverse("instances:update_console", args=[self.instance.id]), data)
+        return wvm.return_value
+
+    def test_console_settings_are_applied(self):
+        proxy = self.post({"listen_on": "127.0.0.1", "password": "", "keymap": "de"})
+        proxy.set_console_keymap.assert_called_once_with("de")
+        proxy.set_console_listener_addr.assert_called_once_with("127.0.0.1")