vhost: add unsafe API to check in-flight packets

vhost: add unsafe API to check in-flight packets

In async data path, when vring state changes or device is destroyed,
it is necessary to know the number of in-flight packets in DMA engine.
This patch provides a thread unsafe API to return the number of
in-flight packets for a vhost queue without using any lock.

Signed-off-by: Xuan Ding <xuan.ding@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>