[libvirt-users] Failed to insert a cd in a virtual machine running

I can't attach a cd when a vm is running with qemu 0.12.3 (ubuntu 10.04), the problem is not with qemu because when I start the vm with kvm command (without libvirt) I can change the cd with qemu-monitor without any problem and eject it too (libvirt say me that it isn't supported by hypervisor). the versions of libvirt that I have tried are 0.7.7 and 0.8.0. Any ideas? Thanks. Jorge