Fix volume XML tests

Run the xml version in the .*XML classes instead of the json version.

Change-Id: Ib7d92119ecf1b1c5ecca8b14747b628c7e70eb7d
diff --git a/tempest/tests/volume/test_volumes_actions.py b/tempest/tests/volume/test_volumes_actions.py
index 155acb6..dd93b89 100644
--- a/tempest/tests/volume/test_volumes_actions.py
+++ b/tempest/tests/volume/test_volumes_actions.py
@@ -22,6 +22,7 @@
 
 
 class VolumesActionsTest(BaseVolumeTest):
+    _interface = "json"
 
     @classmethod
     def setUpClass(cls):