gve: Add DQO RDA support

DQO is the descriptor format for our next generation virtual NIC.
It is necessary to make full use of the hardware bandwidth on many
newer GCP VM shapes.

One major change with DQO from its predecessor GQI is that it uses
dual descriptor rings for both TX and RX queues.

The TX path uses a descriptor ring to send descriptors to HW, and
receives packet completion events on a TX completion ring.

The RX path posts buffers to HW using an RX descriptor ring and
receives incoming packets on an RX completion ring.

In GQI-QPL, the hardware could not access arbitrary regions of
guest memory, which is why there was a pre-negotitated bounce buffer
(QPL: Queue Page List). DQO-RDA has no such limitation.

"RDA" is in contrast to QPL and stands for "Raw DMA Addressing" which
just means that HW does not need a fixed bounce buffer and can DMA
arbitrary regions of guest memory.

A subsequent patch will introduce the DQO-QPL datapath that uses the
same descriptor format as in this patch, but will have a fixed
bounce buffer.

Signed-off-by: Shailend Chand <shailend@google.com>
Reviewed-by: markj
MFC-after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D46690