If we specify block migration, but there are no disks which actually
require block migration we call libvirt's migrateToURI3() with
VIR_MIGRATE_NON_SHARED_INC in flags and an empty migrate_disks in
params. Libvirt interprets this to be the default block migration
behaviour of "block migrate all writeable disks". However,
migrate_disks may only be empty because we filtered attached volumes
out of it, in which case libvirt will block migrate attached volumes.
This is a data corruptor.
This change addresses the issue at the point we call migrateToURI3().
As we never want the default block migration behaviour, we can safely
remove the flag if the list of disks to migrate is empty.
Change-Id: I9b545ca8aa6dd7b41ddea2d333190c9fbed19bc1
Resolves-bug: #1719362
ea9bf5216b