virtio-net: Allow UFO feature to be set and advertised.

- Allow setting UFO on virtio-net and advertise to host.

Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>