examples/vdpa: introduce a new sample for vDPA

examples/vdpa: introduce a new sample for vDPA

The vdpa sample application creates vhost-user sockets by using the
vDPA backend. vDPA stands for vhost Data Path Acceleration which utilizes
virtio ring compatible devices to serve virtio driver directly to enable
datapath acceleration. As vDPA driver can help to set up vhost datapath,
this application doesn't need to launch dedicated worker threads for vhost
enqueue/dequeue operations.

Signed-off-by: Xiaolong Ye <xiaolong.ye@intel.com>
Acked-by: Xiao Wang <xiao.w.wang@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>