virtio: fix received packet size tracepoint

We reported the size of the last buffer in the packet, rather than the size
of the complete packet.

Fix to report the total size.

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