Improve SCSI volume name generation

Improve SCSI volume name generation

The SCSI volumes currently get a name like '17:0:0:1' based
on $host:$bus:$target:$lun. The names are intended to be unique
per pool and stable across pool restarts. The inclusion of the
$host component breaks this, because the $host number for iSCSI
pools is dynamically allocated by the kernel at time of login.
This changes the name to be 'unit:0:0:1', ie removes the leading
host component. The 'unit:' prefix is just to ensure the volume
name doesn't start with a number and make it clearer when seen
out of context.

* src/storage/storage_backend_scsi.c: Improve volume name
  field value stability and uniqueness