To support selective block migration we need to read block devices
from nova block device mappings instead of libvirt block info.
It means that in current implementation we would call
_live_migration_copy_disk_paths two times - from
live_migration_operations and from live_migration_monitor.
To avoid that this change gets disk paths early and pass them as
and additional paremeter to live migration monitor.
Change-Id: Ic894cfc7374ba06b436b2a76a5984012d1dba3a5
Related-bug: #1398999
f0d5fc6191