virtio-net: Rename vnet_txq_stats and vnet_rxq_stats

Rename it to txq_stats and rxq_stats which are more consistent with
struct txq and struct rxq.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>