vhost: fix vid allocation race

vhost: fix vid allocation race

vhost_new_device might be called in different threads at
the same time.

thread 1(config thread)
            rte_vhost_driver_start
               ->vhost_user_start_client
                   ->vhost_user_add_connection
                     -> vhost_new_device

thread 2(vhost-events)
	vhost_user_read_cb
           ->vhost_user_msg_handler (return value < 0)
             -> vhost_user_start_client
                 -> vhost_new_device

So there could be a case that a same vid has been allocated
twice, or some vid might be lost in DPDK lib however still
held by the upper applications.

Another place where race would happen is at the func
*vhost_destroy_device*, but after a detailed investigation,
the race does not exist as long as no two devices have the
same vid: Calling vhost_destroy_devices in different
threads with different vids is actually safe.

Fixes: a277c7159876 ("vhost: refactor code structure")
Cc: stable@dpdk.org
Reported-by: Peng He <hepeng.0320@bytedance.com>
Signed-off-by: Fei Chen <chenwei.0515@bytedance.com>
Reviewed-by: Zhihong Wang <wangzhihong.wzh@bytedance.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>