xen-netfront: always keep the Rx ring full of requests

This is based on Linux commit 1f3c2eba1e2d866ef99bb9b10ade4096e3d7607c from
David Vrabel:

A full Rx ring only requires 1 MiB of memory. This is not enough memory
that it is useful to dynamically scale the number of Rx requests in the ring
based on traffic rates, because:

a) Even the full 1 MiB is a tiny fraction of a typically modern Linux

VM (for example, the AWS micro instance still has 1 GiB of memory).

b) Netfront would have used up to 1 MiB already even with moderate

data rates (there was no adjustment of target based on memory
pressure).

c) Small VMs are going to typically have one VCPU and hence only one

queue.

Keeping the ring full of Rx requests handles bursty traffic better than
trying to converge on an optimal number of requests to keep filled.

Reviewed by: Wei Liu <wei.liu2@citrix.com>
Sponsored by: Citrix Systems R&D