Support usb-net device

We should support qemu's usb-net device. XML something like:

    <interface type='network'>
       [ ... ]
       <model type='usb-net'/>
       <address type='usb' ... />
    </interface>

Previous attempt at patches:

https://www.redhat.com/archives/libvir-list/2013-January/msg00056.html

A good reference commit to see how to implement something like that is this commit adding usb-audio support:

commit e260a0e60a5abc48f27d09900ca3150130919d3f
Author: Peter Krempa <pkrempa@redhat.com>
Date:   Thu Jul 24 17:32:31 2014 +0200

    conf: Add USB sound card support and implement it for qemu