So after a variety of attempts from a number of forum posts, the following approach

link1       

link2

worked for me:

>>  lsscsi

[2:0:0:0] cd/dvd HL-DT-ST BD-RE WH14NS40 1.03 /dev/sr0

And then hand modifying the ubuntu XML VM with 

<controller type='scsi' index='0' model='virtio-scsi'/> 
<hostdev mode='subsystem' type='scsi'>
  <source>
  <adapter name='scsi_host2'/>
  <address type='scsi' bus='0' target='0' unit='0'/>
  source>
  <readonly/> 
  <address type='drive' controller='0' bus='0' target='0' unit='0'/>
hostdev>

Other posts had suggested changing the controller value to "1" which did not work for me. 

I now have access to the Blu-ray drive from within the Ubuntu VM (it automatically detects a Audio disk insert and mounts it).

I am now able to rip audio CDs which was my original objective.

Quelle: https://forums.unraid.net/topic/62070-pass-through-a-host-dvd-drive/?do=findComment&comment=688479