Make sure the first part has the model equalling 'virtio-scsi'. Doing so will allow the vm to recognize the optical drives and install drivers.
...
<controller type='scsi' index='0' model='virtio-scsi'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x0b' function='0x0'/>
</controller>
<controller type='scsi' index='1' model='virtio-scsi'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x0c' function='0x0'/>
</controller>
...
<hostdev mode='subsystem' type='scsi' managed='no'>
<source>
<adapter name='scsi_host2'/>
<address bus='0' target='0' unit='0'/>
</source>
<readonly/>
<address type='drive' controller='1' bus='0' target='0' unit='0'/>
</hostdev>
<hostdev mode='subsystem' type='scsi' managed='no'>
<source>
<adapter name='scsi_host6'/>
<address bus='0' target='0' unit='0'/>
</source>
<readonly/>
<address type='drive' controller='0' bus='0' target='0' unit='0'/>
</hostdev>
...
