only read_done blocks are in the queued to be flushed to the migration
stream. submitted blocks are still in flight.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <1520507908-16743-6-git-send-email-pl@kamp.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>