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.

datasource/views.py

@@ -3,8 +3,11 @@ import socket
 
 from accounts.models import UserInstance, UserSSHKey
 from computes.models import Compute
+from django.core.exceptions import PermissionDenied
 from django.http import Http404, HttpResponse
 from django.shortcuts import get_object_or_404, render
+from instances.models import Instance
+from instances.utils import can_open_console
 from libvirt import libvirtError
 from vrtManager.instance import wvmInstance
 
@@ -95,6 +98,10 @@ def get_vdi_url(request, compute_id, vname):
     :return:
     """
     compute = get_object_or_404(Compute, pk=compute_id)
+    # The VDI URL points straight at the VM's console: same rule as the console.
+    instance = Instance.objects.filter(compute=compute, name=vname).first()
+    if instance is None or not can_open_console(request.user, instance):
+        raise PermissionDenied
 
     try:
         conn = wvmInstance(