[libvirt] Change of CDROM source via virsh

Hi, I would like to ask what is the simplest way, how to change ISO mounted in VM's drive. I am using libvirt+kvm+qemu and only way I found is via virt-manager, but there are some problems with it - it is not working properly on some Fedora testing guests and it disables the VNC password for the VM. I'll try to look to the sources of virt-manager if I could find the cause (I am thinking using dumpxml without SECURE flag) but I would like to change the ISO via script (i.e. via virsh). There must be some simple way but I can not find it... Radek