[Libvir] XM config <-> XML config

Hi, I am using Fedora 7 with virt-manager to control my xen domains. I would like to delegate a pci device to one of my domU's. I know how this is done in the XM and sxp config files, but how is it done now? I read that libvirt can convert XM <-> XML and though I could export my current config to XM format, edit it and reimport. How is is done with libvirt? Thanks, Arik