Vhostuser v3

This is the latest draft of my Snabb Switch app for zero-copy Virtio-net to QEMU guests.

Known issues:
1. VM transmit is getting stuck when its tx ring wraps around (256 packets). Elusive bug.
2. Currently does not test zero-copy receive path (must feed buffers to intel10g app).
3. Many XXX comments in the code that need to be dealt with.

This is a work in progress but now at a fairly advanced stage.

See http://www.virtualopensystems.com/en/solutions/guides/snabbswitch-qemu/ for more background.