Qemu runner

An extension of @titanous's [wip branch](https://github.com/flynn/flynn-test/tree/wip), this supports booting a Flynn cluster using QEMU and running the basic tests against that cluster like:

```
sudo ./flynn-test \
  --cli /path/to/flynn \
  --user `whoami` \
  --rootfs `pwd`/runner/rootfs/rootfs.img \
  --kernel `pwd`/runner/rootfs/vmlinuz
```

To get the benefits of KVM, I have been booting my mac into Ubuntu to run the cluster, where it takes ~5 mins to build all the images, boot 5 Flynn hosts then bootstrap Layer 1.

`sudo` is needed to setup networking and to modify `/etc/hosts` to point domains at strowger.

I think there is room to refactor but opening to get some thoughts on the approach.
Initializing a zero-length slice is rarely necessary, you can usually just leave it `nil`.