[SCSI] iscsi: add transport end point callbacks

add transport end point callbacks so iscsi drivers that cannot connect
from userspace, like iscsi tcp, using sockets do not have to
implement their own socket infrastructure.

Signed-off-by: Or Gerlitz <ogerlitz@voltaire.com>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>