Nimbus/Alamo networking improvements

The network device used by Nimbus virtual machines on Alamo has been changed from Realtek RTL-8139 to Virtio. After this change, our measurements show that network bandwidth to and from Nimbus virtual machines has improved by a factor of 3 or more.   The Virtio device is supported by all of the existing virtual machine images that we tested. Your virtual machines should automatically detect this new device and benefit from the higher networking bandwidth.   If you have any problems with networking on your virtual machines, please submit a ticket.