Set instance['host'] to the original host value on revert resize.

Fixes bug #901445

Change-Id: I8c112d92d6a54715073ef1119372d782c3be37c9

nova/tests/test_compute.py

@@ -838,6 +838,7 @@ class ComputeTestCase(BaseTestCase):
         instance_type_ref = db.instance_type_get(context,
                 inst_ref['instance_type_id'])
         self.assertEqual(instance_type_ref['flavorid'], '1')
+        self.assertEqual(inst_ref['host'], migration_ref['source_compute'])
 
         self.compute.terminate_instance(context, inst_ref['uuid'])