virtio-net: allow increasing rx queue size

This allows increasing the rx queue size up to 1024: unlike with tx,
guests don't put in huge S/G lists into RX so the risk of running into
the max 1024 limitation due to some off-by-one seems small.

It's helpful for users like OVS-DPDK which don't do any buffering on the
host - 1K roughly matches 500 entries in tun + 256 in the current rx
queue, which seems to work reasonably well. We could probably make do
with ~750 entries but virtio spec limits us to powers of two.
It might be a good idea to specify an s/g size limit in a future
version.

It also might be possible to make the queue size smaller down the road, 64
seems like the minimal value which will still work (as guests seem to
assume a queue full of 1.5K buffers is enough to process the largest
incoming packet, which is ~64K).  No one actually asked for this, and
with virtio 1 guests can reduce ring size without need for host
configuration, so don't bother with this for now.

Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Jason Wang <jasowang@redhat.com>
Suggested-by: Patrik Hermansson <phermansson@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
