vhost: fix double free on device stop

vhost dev stop failed to clear the log field.
Typically not an issue as dev start overwrites this field,
but if logging gets disabled before the following start,
it doesn't so this causes a double free.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/vhost.c

@@ -784,5 +784,6 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev)
 
     hdev->started = false;
     qemu_free(hdev->log);
+    hdev->log = NULL;
     hdev->log_size = 0;
 }