gve: Add DQO QPL 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.

This patch extends the previously introduced DQO descriptor format
with a "QPL" mode. QPL stands for Queue Page List and refers to
the fact that the hardware cannot access arbitrary regions of the
host memory and instead expects a fixed bounce buffer comprising
of a list of pages.

The QPL aspects are similar to the already existing GQI queue
queue format: in that the mbufs being input in the Rx path have
external storage in the form of vm pages attached to them; and
in the Tx path we always copy the mbuf payload into QPL pages.

Signed-off-by: Shailend Chand <shailend@google.com>
Reviewed-by: markj
MFC-after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D46691