[gve] Select preferred operating mode

[gve] Select preferred operating mode

Select a preferred operating mode from those advertised as supported
by the device, falling back to the oldest known mode (GQI-QPL) if
no modes are advertised.

Since there are devices in existence that support only QPL addressing,
and since we want to minimise code size, we choose to always use a
single fixed ring buffer even when using raw DMA addressing.  Having
paid this penalty, we therefore choose to prefer QPL over RDA since
this allows the (virtual) hardware to minimise the number of page
table manipulations required.  We similarly prefer GQI over DQO since
this minimises the amount of work we have to do: in particular, the RX
descriptor ring contents can remain untouched for the lifetime of the
device and refills require only a doorbell write.

Signed-off-by: Michael Brown <mcb30@ipxe.org>