tcm_vhost: Enable VIRTIO_SCSI_F_HOTPLUG

Everything for hotplug is ready. Let's enable the feature bit.

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>