DPDK: update dpdk_virtio_to_vm_tx_burst() function to DPDK 2.1
DPDK: update dpdk_virtio_to_vm_tx_burst() function to DPDK 2.1

Change-Id: I7de82279bddd488d325ff14c493504bd928e0735
Closes-bug: 1468430

codilime/contrail-vrouter,codilime/contrail-vrouter,codilime/contrail-vrouter,codilime/contrail-vrouter