[gve] Increase number of receive buffers to reduce packet loss

[gve] Increase number of receive buffers to reduce packet loss

Experiments suggest that using fewer than 64 receive buffers leads to
excessive packet drop rates on some instance types (observed with a
c3-standard-4 instance in europe-west4-a).

Fix by increasing the number of receive data buffers (and adjusting
the length of the registrable queue page address list to match).

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