migration: refactor init_dirty_bitmap_migration

Split out handling one bs, it is needed for the following commit, which
will handle BlockBackends separately.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200521220648.3255-2-vsementsov@virtuozzo.com>
[eblake: shorter subject line]
Signed-off-by: Eric Blake <eblake@redhat.com>
1 file changed