Use virtio network interfaces in virtualbox

The [**virtio**](https://www.virtualbox.org/manual/ch06.html) paravirtualized adapter is well-supported and more efficient than virtualized NICs in VirtualBox. It is the default for boot2docker, for example, and fixes in [CoreOS 459.0.0](https://github.com/coreos/manifest/releases/tag/v459.0.0) make it reliable to use now.

@marineam [had also suggested](https://github.com/coreos/coreos-overlay/pull/900#issuecomment-57751782) that virtio should become the default.