[SCSI] iscsi class: add async scan helper

In qla4xxx's probe it will call the iscsi session setup functions
for session that got setup on the initial start. This then makes
it easy for the iscsi class to export a helper which indicates
when those scans are done.

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