test(instances): isolate the live libvirt suite on shared hosts

- Runs only when TEST_LIBVIRT_HOST is set, so manage.py test on a
  developer machine no longer creates VMs on the local libvirtd.
- Every VM and volume is named wvc-test-*, and volumes live in a
  temporary wvc-test pool (/var/lib/libvirt/wvc-test) instead of the
  default pool next to real images.
- Cleanup removes only prefixed domains and the test pool's volumes;
  it also clears leftovers of an interrupted run before starting, and
  tolerates domains that turned transient.
- tearDownClass fails if any domain or volume outside the test
  namespace appeared, disappeared or was replaced.
- livetest gains create_volume/define_vm helpers and
  test_live_dataloss.py, whose setUpClass opens the class transaction
  last, so a failed libvirt connection cannot lock the test database
  for later test classes.

The first live tests reproduce two known external-snapshot data losses
and are marked expectedFailure: reverting deletes a disk added after
the snapshot, and deleting the older of two snapshots deletes the
newer one's overlay.

README.md

@@ -445,7 +445,7 @@ ruff check .
 ```
 
 > **Live Hypervisor Testing (Optional):**
-> Without these variables the live tests use the local socket (`qemu:///system`) or are skipped. They create and delete `test*` VMs and volumes in the host's `default` pool.
+> The instance tests run only when `TEST_LIBVIRT_HOST` is set. They create `wvc-test-*` VMs and a temporary `wvc-test` pool (`/var/lib/libvirt/wvc-test`), remove them afterwards, and fail if any other VM or volume on the host changed.
 > ```bash
 > export TEST_LIBVIRT_HOST=compute1 TEST_LIBVIRT_TYPE=2   # 1 TCP, 2 SSH, 3 TLS, 4 socket
 > export TEST_LIBVIRT_LOGIN=root TEST_LIBVIRT_PASSWORD=   # login/password for TCP and TLS