iscsi: Add ability to specify or autodetect block vs fileio

When using block devices to back iSCSI logical units it is
advantageous to use blockio as it decreases latency and
increases throughput, effect is especially pronounced with
faster backing storage devices.

Change-Id: Ia8cba5ddfe140cb5732c2b9ad882831e812a44bc
Merge "iscsi: Add ability to specify or autodetect block vs fileio"