Well, you can specify the cd-rom device to boot an HVM from it (see user docs), but AFAIK there's no way to just pass a cd-rom drive through to a VM by itself. (I seem to remember reading a message on here from Joanna implying that it's not possible, but maybe something has changed since then.)
qvm-block -a <vm-name> dom0:sr0
but if I try to write something on the CD it keeps asking to insert CD or
DVD even if it is already inserted.
Best
Franz