libvirt: add comment for vifs_already_plugged=True in finish_migration

Given we unplug VIFs in _cleanup_resize if the host has changed, it's
confusing to pass vifs_already_plugged=True in finish_migration without
some explanation of why we don't check to see if the host has changed
before deciding what the value of vifs_already_plugged should be when
creating the new domain and plugging VIFs again.

After some discussion with Neutron cores, this is the explanation I've
come up with, albeit not a great one, and indicates a gap in some of the
nova/neutron integration story.

Related-Bug: #1323658

Change-Id: I982b6bd0650f29f34c213b6eebbe066f7bd65815

nova/virt/libvirt/driver.py

@@ -6351,6 +6351,11 @@ def finish_migration(self, context, migration, instance, disk_info,
                                   image_meta,
                                   block_device_info=block_device_info,
                                   write_to_disk=True)
+        # NOTE(mriedem): vifs_already_plugged=True here, regardless of whether
+        # or not we've migrated to another host, because we unplug VIFs locally
+        # and the status change in the port might go undetected by the neutron
+        # L2 agent (or neutron server) so neutron may not know that the VIF was
+        # unplugged in the first place and never send an event.
         self._create_domain_and_network(context, xml, instance, network_info,
                                         disk_info,
                                         block_device_info=block_device_info,