[gve] Allow for descriptor and completion lengths to vary by mode

[gve] Allow for descriptor and completion lengths to vary by mode

The descriptors and completions in the DQO operating mode are not the
same sizes as the equivalent structures in the GQI operating mode.
Allow the queue stride size to vary by operating mode (and therefore
to be known only after reading the device descriptor and selecting the
operating mode).

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