tcm_vhost: Flush vhost_work in vhost_scsi_flush()

We also need to flush the vhost_works. It is the completion vhost_work
currently.

Signed-off-by: Asias He <asias@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>