iSCSILogicalUnit: lio-t: Add pscsi LIO-T backing store

With minimal changes LIO-T 'pscsi' backing store can be implemented. As use case example this enables possibility to export tape library robot devices via SCSI passthrough to virtual machine in HA KVM setup where same multiple library robots are zoned to multiple corosync nodes in multiple datacenters via SAN to achieve HA. In such scenario corosync just switch iSCSILogicalUnit to online node. This solution can work with persistent device naming such as '/dev/tape/by-id/<wwn>' but this patch has to be implemented https://github.com/open-iscsi/rtslib-fb/pull/165 because otherwise targetcli returns error when scsi_generic device is being exported via pscsi backend.