examples/vhost: support clear in-flight for async dequeue

examples/vhost: support clear in-flight for async dequeue

This patch allows vring_state_changed() to clear in-flight
dequeue packets. It also clears the in-flight packets in
a thread-safe way in destroy_device().

Signed-off-by: Yuan Wang <yuanx.wang@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Jiayu Hu <jiayu.hu@intel.com>