Ignore dirty submodules so they don't pollute the output of git
status.

The `ignore = dirty` option tells git to disregard any local changes
to the submodule directories. In the case of snabbswitch, the luajit
depenency is installed in the submodule directory itself, which yields
the following `git status` output:

```
kaz@monad:~/src/snabbswitch$ git status
# On branch ignore-dirty-submodules
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#   (commit or discard the untracked or modified content in submodules)
#
#       modified:   deps/luajit (untracked content)
#
no changes added to commit (use "git add" and/or "git commit -a")
```
memory.lua: rename chunk_phys to chunk_phy ; intel.lua: set rdt = 0 in init_receive() , set tdt = 0 in init_transmit()
Merge pull request #47 from rahul-mr/small_fix1

memory.lua: rename chunk_phys to chunk_phy ; intel.lua: set rdt = 0 in init_receive() , set tdt = 0 in init_transmit()
Merge pull request #46 from pkazmier/ignore-dirty-submodules

Ignore dirty submodules so they don't pollute the output of git status
virtio.h: First draft of header file for virtio support.

virtio and its "vring" structure provide a shared memory DMA-like
interface for sending and receiving packets.

We will be able to do two things with virtio:

1. Communicate directly with KVM guests. That is: directly with the device driver running inside the guest VM, without waking up the hypervisor.

2. Communicate with the host Linux kernel to do zero-copy ethernet I/O using the "vhost" architecture (http://blog.vmsplice.net/2011/09/qemu-internals-vhost-architecture.html).

This header is based on the specification:
  virtio: Towards a De-Facto Standard For Virtual I/O Devices (Rusty Russell)
  http://ozlabs.org/~rusty/virtio-spec/virtio-paper.pdf