change default networking to virtio in Ubuntu packaging
rtl8139 is the default network adapter provided in openstack instances.
That is very slow compared to virtio. The only reason you would want it would be for backwards compatibility.
I believe that right now is the time to change that to virtio. If we wait until after 12.04, we may, in 14.04 actually have a large corpus of users who would have broken images by changing that default. Right now there are comparably a very small number of users/amis that would not support virtio.