https://bugzilla.redhat.com/show_bug.cgi?id=1203032

Implement a `migrate_disks' parameters for the QEMU driver. This multi-
value parameter can be used to explicitly specify what block devices
are to be migrated using the NBD server. Tunnelled migration using NBD
is to be done.
Signed-off-by: NPavel Boldin <pboldin@mirantis.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>