virtualbox_ovf.sh: use virtio network interfaces

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.

I exported a CoreOS 490.0.0 VM using virtio from VirtualBox as an .ovf file, compared it to the template, and saw no differences other than the adapter type field, so I believe this is sufficient to default to virtio in virtualbox.

Refs coreos/coreos-vagrant#178.