[SCSI] iscsi class: add endpoint class

Add sysfs representation for the endpoint, so userspace can match the
host and session to the endpoint. This will allow us to set the host's
parent correctly at host creation time.

The next patches will convert tcp and iser, and fix iser's dma_mask
bug.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>