Added vhost archiecture support to Snabb Switch.

This is an optimized method of Ethernet I/O using tap devices -- the
same method that KVM itself uses. It accelerates Snabb Switch's I/O
towards the host and it paves the way for using the same inteface
towards KVM guests.

More information about vhost architecture on Stefan Hajnoczi's blog:
http://blog.vmsplice.net/2011/09/qemu-internals-vhost-architecture.html
memory.lua: Added 'dma_regions' global list of active DMA memory regions.
virtio.lua: Oops forgotten file :)
virtio.lua: Untabify.
Updated values for RX/TXDCTL, RDTR, and RADV to eliminate the MPCs.

Ultimately, these values should be auto-tuned based on performance, or
at the very least, should be configurable. For now, these values are
the same that are used in the e1000e driver so it's probably a safe
starting point for us.