migration: reflect incoming failure to shell

Management apps like libvirt don't know to pay attention to
stderr unless there is a non-zero exit status.

* migration.c (process_incoming_migration_co): Exit with non-zero
status on failure.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 1366149041-626-1-git-send-email-eblake@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>