virtio-net: use if_transmit() instead of legacy if_start()

Switched the virtio-net driver to use if_transmit() instead of legacy
if_start(). This saves us at least 2 additional lock/unlock sequences
per-each mbuf since IF_ENQUEUE() and IF_DEQUEUE() take lock when
pushing/removing the mbuf from the queue if ifnet in a legacy mode.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>