[SCSI] iscsi class: add iface representation

A iscsi host can have multiple interfaces. This patch
adds a new iface iscsi class for this. It exports the
network settings now, and will be extended to also
export iscsi initiator port settings like the isid
and initiator name for drivers that can support multiple
initiator ports.

Based on patch from Lalit Chandivade.

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