fix(storages): keep volumes a VM still needs

libvirt deletes any volume it is asked to. The storage page and the API
deleted a VM's disk, the base image of another volume or the file a
snapshot was taken on without a check, breaking the VM or its
snapshots.

wvmStorage.del_volume now refuses a volume a domain on the compute uses
as a disk or a backing file of one, a file of a domain's snapshots, or
the backing file of another volume. The check is the one destroy
already used, moved to wvmConnect and extended to snapshot files, so
destroy also keeps the files of another VM's snapshots. The storage
page shows the refusal; the API answers 400 with it, as for other
refusals. Deleting a disk from a VM refuses after the detach when the
VM's own snapshots need the volume, and says the volume was kept.

Network disks count by their source name, pool volume disks by their
volume's path. A volume whose metadata cannot be read stops the check
instead of counting as unused; only one removed meanwhile is skipped.
The check and the delete hold the compute lock, so no VM operation
attaches the volume in between; a busy compute is a message on the page
and 409 in the API.

storages/api/viewsets.py

@@ -1,6 +1,7 @@
 from admin.permissions import IsSuperUser
 from appsettings.settings import app_settings
 from computes.models import Compute
+from computes.utils import libvirt_compute_lock
 from django.shortcuts import get_object_or_404
 from drf_spectacular.utils import extend_schema, inline_serializer
 from rest_framework import serializers, status, viewsets
@@ -196,7 +197,9 @@ class VolumeViewSet(viewsets.ViewSet):
         )
 
         if conn.is_active():
-            conn.del_volume(pk)
+            # no VM operation attaches it between the check and the delete
+            with libvirt_compute_lock(compute):
+                conn.del_volume(pk)
             conn.close()
             return Response({"status": "Volume: " + pk + " is deleted"})
         else: