vhost: support clear in-flight packets for async dequeue

vhost: support clear in-flight packets for async dequeue

rte_vhost_clear_queue_thread_unsafe() supports to clear
in-flight packets for async enqueue only. But after
supporting async dequeue, this API should support async dequeue too.

This patch also adds the thread-safe version of this API,
the difference between the two API is that thread safety uses lock.

These APIs maybe used to clean up packets in the async channel
to prevent packet loss when the device state changes or
when the device is destroyed.

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>