tcm_vhost: Multi-target support

In order to take advantages of Paolo's multi-queue virito-scsi, we need
multi-target support in tcm_vhost first. Otherwise all the requests go
to one queue and other queues are idle.

This patch makes:

1. All the targets under the wwpn is seen and can be used by guest.
2. No need to pass the tpgt number in struct vhost_scsi_target to
   tcm_vhost.ko. Only wwpn is needed.
3. We can always pass max_target = 255 to guest now, since we abort the
   request who's target id does not exist.

Changes in v2:
- Handle non-contiguous tpgt

Changes in v3:
- Simplfy lock in vhost_scsi_set_endpoint
- Return -EEXIST when does not match

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