Don't set instance ACTIVE until it's really active

Fixes bug 963656

Reverting a resize would end up setting the instance ACTIVE before the
driver had finished all of the work starting up the original instance.
If the instance is deleted quickly after the revert, a race condition
could occur between restarting the original instance and deleting it.

Change-Id: Iba26ad7d1bc1049137f94e13898be86698963fb3

nova/compute/manager.py

@@ -1226,6 +1226,9 @@ def finish_revert_resize(self, context, instance_uuid, migration_id):
         old_instance_type = migration_ref['old_instance_type_id']
         instance_type = instance_types.get_instance_type(old_instance_type)
 
+        self.driver.finish_revert_migration(instance_ref,
+                                   self._legacy_nw_info(network_info))
+
         # Just roll back the record. There's no need to resize down since
         # the 'old' VM already has the preferred attributes
         self._instance_update(context,
@@ -1239,8 +1242,6 @@ def finish_revert_resize(self, context, instance_uuid, migration_id):
                               vm_state=vm_states.ACTIVE,
                               task_state=None)
 
-        self.driver.finish_revert_migration(instance_ref,
-                                   self._legacy_nw_info(network_info))
         self.db.migration_update(context, migration_id,
                 {'status': 'reverted'})