[libvirt-users] correct way to hot-add cdrom ?

Good day to all. i have problems with cdrom hot adding code. currently i using virDomainAttachDevice with type=file, device=cdrom, dev=hdc, this code works for machine with one ide hdd and one ide cdrom, but this not work for machine with only one ide hdd, and i looking for solution to hot add cdrom to machine independent of existing devices or i need way to determinate which target device can be attached to vm. thanks in advance for any help.