upgrading from 4.4.0-116-generic to latest linux-azure automatically adds an additional ethernet interface

[Jeff Martin]

This behavior is reproducible:
* Create a VM running a generic 16.04 kernel
* update to the linux-azure package
* run ip addr too see the additional interface

This behavior is problematic for azure users wanting to use OMPI because eth0 and eth1 use the same network card but have different IP which will cause OpenMPI job to fail: https://github.com/open-mpi/ompi/issues/5818

Furthermore, it is not expected for a new ethernet interface to be created when installing a kernel.

[Joshua R. Poulson (jrp)]

Did you enable accelerated networking? Accelerated networking enables two interfaces, eth0 will talk to the host via the Hyper-V device driver, and the new interface would be associated with accelerated network, an SRIOV interface to the netork card on the host. Continue to use eth0 as it is the master of both interfaces because they represent the same NIC. If the SRIOV path were to be rescinded the synthetic path will remain to keep your network up and running. The mechanism by which the two interfaces are connected is called "transparent SRIOV".

Ubuntu images in Azure are automatically whitelisted for Accelerated Networking, but the 4.4 kernel does not support it. Linux-azure, however, does.

For more information on Accelerated Networking, see https://docs.microsoft.com/en-us/azure/virtual-network/create-vm-accelerated-networking-cli