[gve] Remove separate concept of "packet descriptor"

[gve] Remove separate concept of "packet descriptor"

The Linux driver occasionally uses the terminology "packet descriptor"
to refer to the portion of the descriptor excluding the buffer
address.  This is not a helpful separation, and merely adds
complexity.

Simplify the code by removing this artifical separation.

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