gve: adminq: DQO specific device descriptor logic

- In addition to TX and RX queues, DQO has TX completion and RX buffer
  queues.
  - TX completions are received when the device has completed sending a
    packet on the wire.
  - RX buffers are posted on a separate queue form the RX completions.
- DQO descriptor rings are allowed to be smaller than PAGE_SIZE.

Signed-off-by: Bailey Forrest <bcf@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Catherine Sullivan <csully@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>