Hello,
I am currently trying to configure of forwarding devices from specific USB ports which is supported by QEMU. But I don't see any support from libvirt side. Due to libvirt security it occurs to be impossible to passthrough devices from specific bus+port combination.
As workaround I may suggest only writing udev rules which use specific script which adds devices to running domains. Unfortunately situation becomes more complicated when I do need to passthrough these devices during system boot -- the special service is required which will handle all these devices after libvirt-guests are up.