libvirt: Provide VIR_MIGRATE_PARAM_PERSIST_XML during live migration

The VIR_MIGRATE_PARAM_PERSIST_XML parameter was introduced in libvirt
v1.3.4 and is used to provide the new persistent configuration for the
destination during a live migration:

https://libvirt.org/html/libvirt-libvirt-domain.html#VIR_MIGRATE_PARAM_PERSIST_XML

Without this parameter the persistent configuration on the destination
will be the same as the original persistent configuration on the source
when the VIR_MIGRATE_PERSIST_DEST flag is provided.

As Nova does not currently provide the VIR_MIGRATE_PARAM_PERSIST_XML
param but does provide the VIR_MIGRATE_PERSIST_DEST flag this means that
a soft reboot by Nova of the instance after a live migration can revert
the domain back to the original persistent configuration from the
source.

Note that this is only possible in Nova as a soft reboot actually
results in the virDomainShutdown and virDomainLaunch libvirt APIs being
called that recreate the domain using the persistent configuration.
virDomainReboot does not result in this but is not called at this time.

The impact of this on the instance after the soft reboot is pretty
severe, host devices referenced in the original persistent configuration
on the source may not exist or could even be used by other users on the
destination. CPU and NUMA affinity could also differ drastically between
the two hosts resulting in the instance being unable to start etc.

As MIN_LIBVIRT_VERSION is now > v1.3.4 this change simply includes the
VIR_MIGRATE_PARAM_PERSIST_XML param using the same updated XML for the
destination as is already provided to VIR_MIGRATE_PARAM_DEST_XML.

NOTE(lyarwood): A simple change to test_migrate_v3_unicode is included
as Iccce0ab50eee515e533ab36c8e7adc10cb3f7019 had removed this from
master.

Co-authored-by: Tadayoshi Hosoya <tad-hosoya@wr.jp.nec.com>
Closes-Bug: #1890501
Change-Id: Ia3f1d8e83cbc574ce5cb440032e12bbcb1e10e98
(cherry picked from commit 1bb8ee95d4)

nova/tests/unit/virt/libvirt/test_driver.py

@@ -11196,6 +11196,7 @@ class LibvirtConnTestCase(test.NoDBTestCase,
             'migrate_disks': disk_paths,
             'bandwidth': _bandwidth,
             'destination_xml': target_xml,
+            'persistent_xml': target_xml,
         }
 
         # start test
@@ -11303,7 +11304,8 @@ class LibvirtConnTestCase(test.NoDBTestCase,
             'migrate_disks': disk_paths,
             'migrate_uri': 'tcp://127.0.0.2',
             'bandwidth': CONF.libvirt.live_migration_bandwidth,
-            'destination_xml': target_xml
+            'destination_xml': target_xml,
+            'persistent_xml': target_xml,
         }
 
         # Start test
@@ -11462,6 +11464,7 @@ class LibvirtConnTestCase(test.NoDBTestCase,
             'migrate_uri': 'tcp://127.0.0.2',
             'bandwidth': CONF.libvirt.live_migration_bandwidth,
             'destination_xml': target_xml,
+            'persistent_xml': target_xml,
         }
 
         # start test
@@ -11813,6 +11816,7 @@ class LibvirtConnTestCase(test.NoDBTestCase,
             'migrate_disks': ['vda', 'vdb'],
             'bandwidth': CONF.libvirt.live_migration_bandwidth,
             'destination_xml': target_xml,
+            'persistent_xml': target_xml,
         }
 
         # start test
@@ -11939,6 +11943,7 @@ class LibvirtConnTestCase(test.NoDBTestCase,
             'migrate_disks': device_names,
             'bandwidth': CONF.libvirt.live_migration_bandwidth,
             'destination_xml': '<xml/>',
+            'persistent_xml': '<xml/>',
         }
         if not params['migrate_disks']:
             del params['migrate_disks']
@@ -12078,6 +12083,7 @@ class LibvirtConnTestCase(test.NoDBTestCase,
             'migrate_disks': disk_paths,
             'bandwidth': CONF.libvirt.live_migration_bandwidth,
             'destination_xml': '<xml/>',
+            'persistent_xml': '<xml/>',
         }
 
         # Prepare mocks

nova/tests/unit/virt/libvirt/test_guest.py

@@ -682,6 +682,7 @@ class GuestTestCase(test.NoDBTestCase):
                 'an-uri', flags=1, params={'migrate_uri': 'dest-uri',
                                            'migrate_disks': 'disk1',
                                            'destination_xml': '</xml>',
+                                           'persistent_xml': '</xml>',
                                            'bandwidth': 2})
 
     @testtools.skipIf(not six.PY2, 'libvirt python3 bindings accept unicode')
@@ -699,6 +700,7 @@ class GuestTestCase(test.NoDBTestCase):
                                            'migrate_disks': ['disk1',
                                                              'disk2'],
                                            'destination_xml': expect_dest_xml,
+                                           'persistent_xml': expect_dest_xml,
                                            'bandwidth': 2})
 
     def test_abort_job(self):

nova/virt/libvirt/guest.py

@@ -638,6 +638,7 @@ class Guest(object):
 
         if destination_xml:
             params['destination_xml'] = destination_xml
+            params['persistent_xml'] = destination_xml
         if migrate_disks:
             params['migrate_disks'] = migrate_disks
         if migrate_uri: