how to check if hal is ready

[Artem Kachitchkine]

Thinking further, we have the Rescan() method, which can be invoked on a 
device. When invoked on a storage device, Rescan() will discover new 
volume and remove stale volumes (e.g. if a disk was reformatted or media 
replaced). It has to have a well-define behavior, and while the method 
returns before rescan is completed, I think the rescan process should be 
finite, i.e. it can't run indefinitely. Perhaps we could have something 
like "info.scan_state" property, with values 'in_progress' and 'done'.

-Artem.