If the target's unit directory contains a Unit_Unique_ID entry, we
should use that as the target's GUID for identification purposes. The
SBP-2 standards document says:
"Although the node unique ID (EUI-64) present in the bus information
block is sufficient to uniquely identify nodes attached to Serial Bus,
it is insufficient to identify a target when a vendor implements a
device with multiple Serial Bus node connections. In this case initiator
software requires information by which a particular target may be
uniquely identified, regardless of the Serial Bus access path used."