net: log packets going through loopback and virtio-net.

There was no way to sniff packets going through OSv's loopback
interface. I faced a need to debug in-guest TCP traffic. Packets are
logged using tracing infrastructure. Packet data is serialized as
sample data up to a limit, which is currently hardcoded to 128 bytes.

To enable capturing of packets just enable tracepoints named:
  - net_packet_loopback
  - net_packet_eth

Raw data can be seen in `trace list` output. Better presentation
methods will be added in the following patches.

This may also become useful when debugging network problems in the
cloud, as we have no ability to run tcpdump on the host there.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>