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.
Merge pull request #205 from JOJ0/master

Medium: iSCSILogicalUnit: add lio_iblock parameter

Setting this parameter to different values prevents race when multiple resources are starting in parallel on the same host. According to the commiter:

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.