target: Add device attribute to expose config_item_name for INQUIRY model

This patch changes LIO to use the configfs backend device name as the
model if you echo '1' to an individual device's emulate_model_alias attribute.
This is a valid operation only on devices with an export count of 0.

Signed-off-by: Tregaron Bayly <tbayly@bluehost.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>