qdisk: fix block size check
qdisk: fix block size check

When using device="" instead of label="", this check was causing
qdiskd to incorrectly exit.

Resolves: #470533

stevenraspudic/resource-agents,asp24/resource-agents,stevenraspudic/resource-agents,asp24/resource-agents,asp24/resource-agents,stevenraspudic/resource-agents