Fix possible race condition while caching: dependencies not yet cached
Fix possible race condition while caching: dependencies not yet cached

If a property is dependent on another device that has not yet been
cached, the caching process fails. This is now fixed.

coldfix/udiskie,pstray/udiskie,pstray/udiskie,khardix/udiskie,mathstuf/udiskie,coldfix/udiskie