Fix wrong assert methods

Fix wrong assert methods in the unit tests for libvirt driver.

Change-Id: I804d504d21953a936a18b76334557fe3ccb97107
Closes-Bug: #1833182

Author: natsumetakashi

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

@@ -10104,7 +10104,7 @@ def _test_direct(migrate, has_min_version,
                 migrate_data, guest, [])
             get_updated_guest_xml.assert_called_once()
             migrate.assert_called()
-            detach.asset_called()
+            detach.assert_called()
 
         _test_direct()
 
@@ -10123,7 +10123,7 @@ def _test_mix(migrate, has_min_version, get_updated_guest_xml, detach):
                 migrate_data, guest, [])
             get_updated_guest_xml.assert_called_once()
             migrate.assert_called()
-            detach.asset_called_once()
+            detach.assert_called_once()
 
         _test_mix()