virtio_net: Factor out the logic to determine xdp sq

Make sure to use the same logic in all places to determine xdp sq. This
is useful for xdp counters which the following commit will introduce as
well.

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>