tcm_vhost: Add vhost_scsi_send_bad_target() helper

Share the send bad target code with other use cases.

Signed-off-by: Asias He <asias@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>