Use disk_bus when guessing the device name for vol

Even though the disk bus could be set through hw_disk_bus property,
the libvirt driver didn't guess the proper bus and device name for the
root volume and expected it to be defined in the BDM.

With this patch, we consider the predefined disk_bus when going to guess
the volume device name and bus instead of just relying on the one
defined in the BDM.

Change-Id: Ic6434ee2b29dc20126ec3b470d7340a0f8bf62e9
Closes-Bug: 1281989
Merge "Use disk_bus when guessing the device name for vol"