feat(Vagrantfile): use "virtio" network devices by default

When having problems with the default Intel NICs in vagrant, we previously tried to use the [**virtio**](https://www.virtualbox.org/manual/ch06.html) paravirtualized adapter but ended up with an AMD card. The issue then was that we couldn't get a DHCP address reliably; this seems fixed now as of https://github.com/coreos/manifest/releases/tag/v459.0.0.

We didn't track a specific issue for this in deis. The CoreOS changes appear to be these:
- https://github.com/coreos/coreos-overlay/pull/900
- https://github.com/coreos/coreos-overlay/pull/904