examples/vhost: use burst enqueue and dequeue from lib

In switch_worker and virtio_tx_local, rte_vhost_enqueue_burst is called to
push host packets to guest VM.
Before enqueue packets to guest VM, vhost example uses configure-able retry logic
to wait for enough vring entries.
In switch_worker, rte_vhost_dequeue_burst is called to get packets from guest VM,
then virtio device will be bound to a queue in VMDQ for the first transmitted
packet.

Signed-off-by: Huawei Xie <huawei.xie@intel.com>
Acked-by: Changchun Ouyang <changchun.ouyang@intel.com>