[SCSI] iscsi class: add iscsi host set param event

The iscsi class uses the set_param event to set session
and connection params. This patch adds a set_host_param
so we can set host level values.

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