fix: add an authorization matrix test and close the endpoints it finds

Add webvirtcloud/test_authorization_matrix.py, which walks every named URL
and checks that a user without any VM/compute relation only reaches an
explicit allowlist, and that the global view_instances permission does not
open consoles. It found:

- /computes/<id>/... views without any check (compute_graph,
  get_compute_*, get_dom_capabilities, storages.get_volumes): the whole
  computes URL namespace is now wrapped with superuser_only, so a view
  added there later cannot be exposed by a missing decorator.
- guess_mac_address (leaks dhcpd.conf host/MAC pairs), random_mac_address
  and guess_clone_name: superuser-only, like the buttons that use them.
- check_instance: cross-tenant VM name lookup with no caller; removed.
- getvvfile and get_vdi_url: reachable with only view_instances, and the
  .vv file contains the VNC password.

Console access now follows one rule, instances.utils.can_open_console:
an active superuser or an active owner of the VM. The global
view_instances permission is read-only. It is used by the console page,
novncd, getvvfile and get_vdi_url (getvvfile and the VDI tab are kept for
virt-viewer users). The console page answers 403 instead of 500 when
access is denied, with the same answer for a missing VM.

console/views.py

@@ -1,14 +1,14 @@
 # pylint: disable=no-name-in-module,no-member
 import re
 
-from accounts.models import UserInstance
 from appsettings.settings import app_settings
 from django.conf import settings
 from django.core.signing import BadSignature, SignatureExpired, TimestampSigner
-from django.http.response import HttpResponseServerError
+from django.http.response import HttpResponseForbidden
 from django.shortcuts import render
 from django.utils.translation import gettext_lazy as _
 from instances.models import Instance
+from instances.utils import can_open_console
 from libvirt import libvirtError
 from vrtManager.instance import wvmInstance
 
@@ -79,24 +79,16 @@ def console(request):
         if host is None or uuid is None:
             raise ValueError("Invalid token")
 
-        if not request.user.is_superuser and not request.user.has_perm(
-            "instances.view_instances"
-        ):
-            try:
-                userInstance = UserInstance.objects.get(
-                    instance__compute_id=host,
-                    instance__uuid=uuid,
-                    user__id=request.user.id,
-                )
-                instance = Instance.objects.get(compute_id=host, uuid=uuid)
-            except UserInstance.DoesNotExist:
-                instance = None
-                console_error = _(
-                    "User does not have permission to access console or host/instance not exist"
-                )
-                return HttpResponseServerError(console_error)
-        else:
-            instance = Instance.objects.get(compute_id=host, uuid=uuid)
+        instance = Instance.objects.filter(compute_id=host, uuid=uuid).first()
+        if instance is None and request.user.is_superuser:
+            raise Instance.DoesNotExist
+        # Same answer for "no such VM" and "not allowed", so the response
+        # does not reveal whether a VM exists.
+        if instance is None or not can_open_console(request.user, instance):
+            console_error = _(
+                "User does not have permission to access console or host/instance not exist"
+            )
+            return HttpResponseForbidden(console_error)
 
         # Generate secure signed token for noVNC daemon
         token = signer.sign(f"{host}:{uuid}:{request.user.id}")