Return early on device_owner=network:dhcp filter

These ports are not present in contrail so we
can return an empty list very early in the
list method.

Change-Id: Icd9dd1c23ead3ead4a628ff504b48bcb8daa7f8c
Closes-Bug: #1559592

neutron_plugin_contrail/plugins/opencontrail/vnc_client/vmi_res_handler.py

@@ -950,6 +950,10 @@ def get_vmi_list(self, **kwargs):
         return self._resource_list(**kwargs)
 
     def resource_list(self, context=None, filters=None, fields=None):
+        if (filters.get('device_owner') == 'network:dhcp' or
+                'network:dhcp' in filters.get('device_owner', [])):
+            return []
+
         if not context:
             context = {'is_admin': True}