libvirt users - Jun 2011 - Bypass USB to Xen VM

Hi. I'm trying to bypass an USB device from the host to a paravirtualized
Xen VM.
I found this page:
http://libvirt.org/formatdomain.html#elementsUSB
which says how to bypass usb and pci devices to guests. It says that works on
KVM only, but is that notice about PCI or about ALL?

Anyway I tried both from virt-manager and from virsh edit domain by adding the
following:

    <hostdev mode='subsystem' type='usb'>
      <source>
        <vendor id='0x03f0'/>
        <product id='0x0114'/>
      </source>
    </hostdev>

but the edit is ignored, right after I save, if I do a dumpxml the
"hostdev" stanza is not present.

Why? Any other way to bypass? usbip?

thanks