Performance v2

This is a set of performance related patches.
It can be split in 2 parts:

1. Optimize designs/bench/basic1
 - make 'debug' dependent asserts/checks on the data path
 - remove ffi.fill in packet.free. This may affect other modules. They should be fixed to not depend on zeroed iovecs.
 - 

2. Optimize virtio throughput
 - small virt to phys optimization
 - small map from guest optimization

Performance numbers are not stable and depend on the overall machine load. So here will put the maximum seen:

- sudo numactl --cpunodebind=1 --membind=1 ./src/snabb ./src/designs/bench/basic1 10e7
upto 6.8 Mpps

- iperf -c
upto 5Gbps