Add options for tuning segment offloading:
<driver>
<host csum='off' gso='off' tso4='off' tso6='off'
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.
