host: Use a custom libvirt network

This was previously implemented in #229 but caused issues when re-bootstrapping a cluster. In the previous change, bootstrapping for the second time would get stuck at `wait router-wait` as the router would crash with the following error:

```
2014/09/22 21:53:05 501: All the given peers are not reachable (Tried to connect to each peer twice and failed) [0]
```

After modifying the change to explicitly set the MAC address of the `flynnbr0` interface everytime (rather than previously only setting it when we creating the bridge, not on subsequent invocations of `flynn-host`), this no longer happens and I can bootstrap the cluster many times.

I can't pin down the underlying issue, but this change has fixed it :expressionless: