gve: Add basic driver framework for Compute Engine Virtual NIC

Add a driver framework for the Compute Engine Virtual NIC that will be
available in the future.

At this point the only functionality is loading the driver.

Signed-off-by: Catherine Sullivan <csully@google.com>
Signed-off-by: Sagi Shahar <sagis@google.com>
Signed-off-by: Jon Olson <jonolson@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Luigi Rizzo <lrizzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>