rte_vhost_enqueue_burst copies host packets to guest.
rte_vhost_enqueue_burst will call virtio_dev_rx and virtio_dev_merge_rx
respectively depending on whether merge-able feature is negotiated or not
in the vhost device.
virtio_dev_merge_tx is renamed to rte_vhost_dequeue_burst.
rte_vhost_dequeue_burst gets to-be-sent packets from guest.