[libvirt-users] 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