libvirt: Can I assign a USB device to a specific USB Controller?

Hi.

Running RHEL6 and Win XM VMs.

I've just figured out how to add a ich9-ehci USB controller. This was done in order to get a USB2 Audio device to work correctly.

Now I have created a uhci controller and want to point a different USB device to that controller, as this other device fails on the new ehci controller. The trouble is the

address> field only seems to support 'bus' and 'device'.

Unfortunately I can't dump any xml files or anything to this, but would appreciate being guided in the right direction.

Thankyou.
Owen.