conf: add options for disabling segment offloading
    
    Add options for tuning segment offloading:
    <driver>
      
            ecn='off' ufo='off'/>
      <guest csum='off' tso4='off' tso6='off' ecn='off' ufo='off'/>
    </driver>
    which control the respective host_ and guest_ properties
    of the virtio-net device.