[CentOS-virt] Can't get USB printer visible in guest

I'm trying to get a USB printer which works on the host (CentOS 5.2 x86_64) to work on a guest (also CentOS 5.2 x86_64).

I'm reading Running Xen at the moment, it is a great starting point that brings together all manner of information in one coherent place, but I'm finding the section on Device Virtualisation a bit unclear (or lacking detail).

As I read it, to create a USB device for a HVM machine I need

device_model = "/usr/lib64/xen/bin/qemu-dm" usb = 1

in the guest's config file. There is not much more specified in the text than this.

I've done this but without success. When I go to add a device to the guest in the GUI tool, I still don't get an option for any kind of USB device.

What have I missed?