net/virtio: support in-order Rx and Tx

net/virtio: support in-order Rx and Tx

IN_ORDER Rx function depends on merge-able feature. Descriptors
allocation and free will be done in bulk.

Virtio dequeue logic:
    dequeue_burst_rx(burst mbufs)
    for (each mbuf b) {
            if (b need merge) {
                    merge remained mbufs
                    add merged mbuf to return mbufs list
            } else {
                    add mbuf to return mbufs list
            }
    }
    if (last mbuf c need merge) {
            dequeue_burst_rx(required mbufs)
            merge last mbuf c
    }
    refill_avail_ring_bulk()
    update_avail_ring()
    return mbufs list

IN_ORDER Tx function can support offloading features. Packets which
matched "can_push" option will be handled by simple xmit function. Those
packets can't match "can_push" will be handled by original xmit function
with in-order flag.

Virtio enqueue logic:
    xmit_cleanup(used descs)
    for (each xmit mbuf b) {
            if (b can inorder xmit) {
                    add mbuf b to inorder burst list
                    continue
            } else {
                    xmit inorder burst list
                    xmit mbuf b by original function
            }
    }
    if (inorder burst list not empty) {
            xmit inorder burst list
    }
    update_avail_ring()

Signed-off-by: Marvin Liu <yong.liu@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>