If a migration is already in progress and somebody attempts
to add a migration blocker, this should rightly fail.

Add an errp parameter and a retcode return value to migrate_add_blocker.
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NAshijeet Acharya <ashijeetacharya@gmail.com>
Message-Id: <1484566314-3987-5-git-send-email-ashijeetacharya@gmail.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Acked-by: NGreg Kurz <groug@kaod.org>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
  Merged with recent 'Allow invtsc migration' change