Low: iSCSILogicalUnit: add lio_iblock parameter. 

When having multiple LUNs using the same iblock device, and they're
started simultaneously, the tcm_core script might see a race condition
on mkdir() for the "/sys/kernel/config/target/core/iblock_0" directory,
preventing at least one instance from starting.

Make the iblock ID configurable to avoid the name clash.