This is a simple placeholder implementation for an optimized
bit-blitting API.

Update the vhost-user code to perform Snabb<->VM memory copies via the
lib.blit module. This allows experimental optimizations with local
changes to the blit module.

This essentially separates "virtio vring processing" and "virtio
memory copies" into being two separate problems that can be profiled
and optimized separately.

This is work-in-progress: Care must be taken not to let the guest see
that packets are available until the blit.barrier() operation has been
executed and I think this will require moving the ring index updates.

work in progress / not complete:
- Rounds all copies up to 32-bytes.
- Fails NFV benchmark test.

The lib.blit API is now implemented by an assembler routine that
batches copies together.

This is a work in progress due to one major restriction: copy length
has to be a multiple of 32 bytes.