virtio: support UDP fragmentation offload (UFO)

Indicate to the virtual hardware that our stack supports UDP fragmentation
offload.

This improves performance by a factor of about 3.3 (from ~20Gbps to ~66Gbps)
running the netperf UDP STREAM test.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>