Comment 2 for bug 1489175

I had to use "virsh edit <domain>" to manually add an <address> specifier thus:

$ virsh dumpxml Kubuntu14.04
--- 8-< snip ---
    <hostdev mode='subsystem' type='usb' managed='yes'>
      <source>
        <vendor id='0x8564'/>
        <product id='0x1000'/>
        <address bus='2' device='7'/>
      </source>
      <alias name='hostdev0'/>
    </hostdev>
--- 8-< snip ---