dm: enhence the mevent API

There is one race issue between mevent callback (which is called
in mevent_dispatch thread) and mevent_delete (which could be called
in dev thread). And the callback is called after mevent_delete.

libevent have the exactly same issue. The issue is decripted here:
https://github.com/libevent/libevent/blob/master/whatsnew-2.1.txt

The fixing is:
We introduce a teardown callback to mevent and make sure there is
no race issue between callback and teardown call.

This patch updates the mevent API and the caller as well.

Tracked-On: #1877
Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
Acked-by: Yu Wang <yu1.wang@intel.com>

devicemodel/hw/pci/virtio/vhost.c

@@ -283,7 +283,7 @@ vhost_vq_register_eventfd(struct vhost_dev *vdev,
 		 */
 		if (is_register) {
 			vq->mevp = mevent_add(vq->call_fd, EVF_READ,
-				vhost_vq_notify, vq);
+				vhost_vq_notify, vq, NULL, NULL);
 			if (!vq->mevp) {
 				WPRINTF("mevent_add failed\n");
 				rc = -1;