net/virtio: fix wrong Rx/Tx method for secondary process

net/virtio: fix wrong Rx/Tx method for secondary process

If the primary enables the vector Rx/Tx path, the current code would
let the secondary always choose the non vector Rx/Tx path. This results
to a Rx/Tx method mismatch between primary and secondary process. Werid
errors then may happen, something like:

    PMD: virtio_xmit_pkts() tx: virtqueue_enqueue error: -14

Fix it by choosing the correct Rx/Tx callbacks for the secondary process.
That is, use vector path if it's given.

Fixes: 8d8393fb1861 ("virtio: pick simple Rx/Tx")
Cc: stable@dpdk.org
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>