target: export protocol identifier

I think the transport statistics device file was supposed
to show scsiTransportProtocolType. It instead shows the
fabric name which is normally closer to the driver name.

I was thinking I cannot change from fabric name to protocol
type name incase people are expecting the driver name, so
this patch adds another file proto_id that exports the SCSI
protocol identifier ID.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>