fix(api): a VM action answers with what the page would have shown after it

The API's VM actions call the web views and only looked at the status
code; the views report a partial result (a disk that was not deleted or
was kept, a template that was not started) as a message for the page
they redirect to. The API reported plain success and left the messages in
the session. Its answer now carries them in "messages".

webvirtcloud/serializers.py

@@ -5,3 +5,10 @@ class StatusSerializer(serializers.Serializer):
     """Response body of API actions that only report a status message."""
 
     status = serializers.CharField()
+
+
+class ActionStatusSerializer(StatusSerializer):
+    """A VM action's status and what the web page would have shown after it,
+    such as a disk that was kept or a template that was not started."""
+
+    messages = serializers.ListField(child=serializers.CharField())