Input XML snip:
<redirdev bus='usb' type='spicevmc'>
   <address type='usb' bus='0' port='4'/>
 </redirdev>
<redirfilter>
  <usbdev class='0x08' vendor='0x1234' product='0xbeef' \
          version='2.00' allow='yes'/>
  <usbdev class='-1' vendor='-1' product='-1' version='-1' allow='no'/>
</redirfilter>

will be converted to:
-device usb-redir,chardev=charredir0,id=redir0,\
filter=0x08:0x1234:0xBEEF:0x2000:1|-1:-1:-1:-1:0,bus=usb.0,port=4