Add gve, the driver for Google Virtual NIC (gVNIC)

gVNIC is a virtual network interface designed specifically for
Google Compute Engine (GCE). It is required to support per-VM Tier_1
networking performance, and for using certain VM shapes on GCE.

The NIC supports TSO, Rx and Tx checksum offloads, and RSS.
It does not currently do hardware LRO, and thus the software-LRO
in the host is used instead. It also supports jumbo frames.

For each queue, the driver negotiates a set of pages with the NIC to
serve as a fixed bounce buffer, this precludes the use of iflib.

Reviewed-by: markj
MFC-after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D39873