Fix another error handling case

The return from this error path is a dict, but the actual return value
(on the non-error handling) is a list of dicts. Change accordingly.

Reviewed-by: imsnah

lib/backend.py

@@ -393,7 +393,7 @@ def NodeVolumes():
   if result.failed:
     logging.error("Failed to list logical volumes, lvs output: %s",
                   result.output)
-    return {}
+    return []
 
   def parse_dev(dev):
     if '(' in dev: