storage: Fetch a unique key for vHBA/NPIV LUNs

https://bugzilla.redhat.com/show_bug.cgi?id=1657468

Commit be1bb6c95 changed the way volumes were stored from a forward
linked list to a hash table. In doing so, it required that each vol
object would have 3 unique values as keys into tables - key, name,
and path. Due to how vHBA/NPIV LUNs are created/used this resulted
in a failure to utilize all the LUN's found during processing.

During virStorageBackendSCSINewLun processing fetch the key (or
serial value) for NPIV LUN's using virStorageFileGetNPIVKey which
will formulate a more unique key based on the serial value and
the port for the LUN.

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>