Fix send-to-zfs crash when VM has a detached CDROM

[lowell80]

• Silently skip storage volume path of 'none'

Example of a problematic config:

root@proxmox-node01:~# qm config 104 | grep ide
boot: order=virtio0;ide2;net0
ide2: none,media=cdrom

This would run the command pvesm path none which would produce the output:

400 Parameter verification failed.
volume: invalid format - unable to parse volume ID 'none'

Simply skipping "none" devices should prevent this. While looking for media=cdrom may also work, this seemed like the most direct approach without having to remember to add backup=0 to all VMs.