Fix some collecton validation

And add restrict_level helper method.

modules/nova/backend.py

@@ -64,8 +64,7 @@ class NovaBackend(BaseBackend, ICapRadio, ICapCollection):
 
     def iter_resources(self, objs, split_path):
         if Radio in objs:
-            if len(split_path) > 0:
-                raise CollectionNotFound(split_path)
+            self._restrict_level(split_path)
 
             for id in self._RADIOS.iterkeys():
                 yield self.get_radio(id)