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/virtio_net.c

@@ -707,7 +707,7 @@ virtio_net_tap_setup(struct virtio_net *net, char *devname)
 
 	if (vhost_fd < 0) {
 		net->mevp = mevent_add(net->tapfd, EVF_READ,
-				       virtio_net_rx_callback, net);
+				       virtio_net_rx_callback, net, NULL, NULL);
 		if (net->mevp == NULL) {
 			WPRINTF(("Could not register event\n"));
 			close(net->tapfd);