Pass block device info in pre_live_migration

In case of block migration, this adds block device information to the
disk information handled by pre_live_migration, so that at least in the
case of libvirt no spurious files are created corresponding to volumes.

Co-Authored-By: florent.flament@cloudwatt.com
Change-Id: I373001e0ef0e4fe4ab900d399756f27101cfe5c8
Closes-Bug: 1376586
Merge "Pass block device info in pre_live_migration"