how to check if hal is ready

[Artem Kachitchkine]

I think he means that he needs to ensure there are 2 volumes on one storage 
device. If you register for volume add events, how long should you wait for the 
2nd one? After adding one volume to GDL, HAL might still be probing more 
volumes. According to the HAL spec, there used to the a property 
"block.have_scanned" that might help, but I don't think it's implemented anymore.

-Artem.