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.

instances/livetest.py

@@ -0,0 +1,127 @@
+"""Safety helpers for the live libvirt test suite.
+
+Everything the suite creates is named with PREFIX, and its volumes live
+in a dedicated pool. Cleanup only removes prefixed domains and the
+volumes of that pool, so a broken test cannot delete other VMs or images
+on a shared host. inventory() lets a suite check that nothing else
+changed.
+"""
+
+import os
+
+import libvirt
+
+PREFIX = "wvc-test-"
+POOL = "wvc-test"
+POOL_PATH = "/var/lib/libvirt/wvc-test"
+
+UNDEFINE_FLAGS = (
+    libvirt.VIR_DOMAIN_UNDEFINE_MANAGED_SAVE
+    | libvirt.VIR_DOMAIN_UNDEFINE_SNAPSHOTS_METADATA
+    | libvirt.VIR_DOMAIN_UNDEFINE_NVRAM
+    | libvirt.VIR_DOMAIN_UNDEFINE_CHECKPOINTS_METADATA
+)
+
+
+def enabled():
+    """The suite creates and deletes VMs, so it only runs on an explicitly named host."""
+    return bool(os.environ.get("TEST_LIBVIRT_HOST"))
+
+
+def ensure_pool(conn):
+    try:
+        pool = conn.storagePoolLookupByName(POOL)
+    except libvirt.libvirtError:
+        pool = conn.storagePoolDefineXML(
+            f"<pool type='dir'><name>{POOL}</name><target><path>{POOL_PATH}</path></target></pool>"
+        )
+        pool.build(0)
+    if not pool.isActive():
+        pool.create(0)
+    pool.refresh(0)
+    return pool
+
+
+def cleanup(conn):
+    """Remove prefixed domains and every volume of the test pool."""
+    for dom in conn.listAllDomains():
+        if not dom.name().startswith(PREFIX):
+            continue
+        persistent = dom.isPersistent()
+        if dom.isActive():
+            dom.destroy()
+        if persistent:
+            dom.undefineFlags(UNDEFINE_FLAGS)
+    try:
+        pool = conn.storagePoolLookupByName(POOL)
+    except libvirt.libvirtError:
+        return
+    if pool.isActive():
+        pool.refresh(0)
+        for vol in pool.listAllVolumes():
+            vol.delete(0)
+
+
+def remove_pool(conn):
+    cleanup(conn)
+    try:
+        pool = conn.storagePoolLookupByName(POOL)
+    except libvirt.libvirtError:
+        return
+    if pool.isActive():
+        pool.destroy()
+    pool.delete(0)
+    pool.undefine()
+
+
+def inventory(conn):
+    """Domains and volumes outside the test namespace."""
+    domains = {(d.name(), d.UUIDString()) for d in conn.listAllDomains() if not d.name().startswith(PREFIX)}
+    volumes = set()
+    for pool in conn.listAllStoragePools():
+        if pool.name() == POOL or not pool.isActive():
+            continue
+        pool.refresh(0)
+        volumes.update((pool.name(), v.name()) for v in pool.listAllVolumes())
+    return domains, volumes
+
+
+def create_volume(conn, name, size_mib=64):
+    """A qcow2 volume in the test pool; returns its path."""
+    assert name.startswith(PREFIX)
+    pool = conn.storagePoolLookupByName(POOL)
+    vol = pool.createXML(
+        f"<volume><name>{name}.qcow2</name><capacity unit='MiB'>{size_mib}</capacity>"
+        "<target><format type='qcow2'/></target></volume>",
+        0,
+    )
+    return vol.path()
+
+
+OVMF_CODE = "/usr/share/edk2/ovmf/OVMF_CODE.fd"
+
+
+def define_vm(conn, name, disk_paths, uefi=False):
+    """A small q35 VM without an OS; disks are attached as vda, vdb, ..."""
+    assert name.startswith(PREFIX)
+    disks = "".join(
+        f"<disk type='file' device='disk'><driver name='qemu' type='qcow2'/>"
+        f"<source file='{path}'/><target dev='vd{chr(ord('a') + i)}' bus='virtio'/></disk>"
+        for i, path in enumerate(disk_paths)
+    )
+    firmware = (
+        "<os><type arch='x86_64' machine='q35'>hvm</type>"
+        f"<loader readonly='yes' type='pflash'>{OVMF_CODE}</loader></os>"
+        if uefi
+        else "<os><type arch='x86_64' machine='q35'>hvm</type></os>"
+    )
+    return conn.defineXML(
+        f"<domain type='kvm'><name>{name}</name><memory unit='MiB'>128</memory><vcpu>1</vcpu>"
+        f"{firmware}<features><acpi/></features><devices>{disks}</devices></domain>"
+    )
+
+
+def volume_exists(conn, path):
+    pool = conn.storagePoolLookupByName(POOL)
+    pool.refresh(0)
+    return os.path.basename(path) in pool.listVolumes()