previously, we did not check the file parameter of a disk,
allowing passthrough of a block device (by design)
with the change to the json parser for the disks, the format
became 'pve-volume-id' which is only valid for our volume ids
(and later we also allowed the value 'none')
this patch alternatively checks if the parameter is a path
or 'cdrom'
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>