Implement l2fwd-like app

[dpino]

A new attempt at implementing this feature. Fixes https://github.com/Igalia/snabb/issues/209

First thing, I think we don't need a copycat of DPDK's l2fwd. Basically what we need is the ability of forward packets to a port and see how many are back. Useful for checking performance of a VM, for example.

So I implemented this feature in two parts:

• A forwarder app, that simply forwards packets received in a port to its origin.
• A new mode in packetblaster (l2fwd). Basically it works like the replay mode but it reports average aggregated statistics.

In this manner, we can run the l2fwd app inside the VM and the host, running packetblaster, will print out the stats (easier to collect).

Example of use:

$ sudo taskset -c 2 ./snabb l2fwd --pciaddr 02:00.0

$ sudo taskset -c 11 ./snabb packetblaster l2fwd from-inet-and-b4-500.pcap 82:00.0
Warming up for 15 seconds...Done
Running for 1 seconds
Device: 82:00.0
Packets sent: 2.3148 MPPS (9.6294 Gbps)
Packets received: 2.3148 MPPS (9.6294 Gbps)
Packet difference: 0

It's pending to support virtio and tap interfaces in the l2fwd app and test it.

[dpino]

Added support for VirtioNet driver. Performance is good. Beware of binding SnabbNFV to the right NUMA node.

(guest) $ sudo ./snabb l2fwd --virtio 0000:00:08.0

(host) $ $ sudo taskset -c 2 ./snabb packetblaster l2fwd from-inet-and-b4-500.pcap 0000:02:00.0                                                      
Warming up for 15 seconds...Done            
Running for 1 seconds
Device: 02:00.0
Packets sent: 2.3149 MPPS (9.6293 Gbps)
Packets received: 2.2672 MPPS (9.4311 Gbps)
Packet difference: 47,680

[dpino]

Should I add the scripts to run SnabbNFV and QEMU somewhere? (perhaps in program/l2fwd/scripts).