vhost: initialize postcopy ufd properly

Currently, postcopy_ufd is initialized to 0 implicitly, so fd 0
could be closed unexpectedly by vhost_backend_cleanup(). Fix this
issue by initializing postcopy_ufd to -1 explicitly.

Fixes: 9eefef3b5970 ("vhost: introduce postcopy advise message")

Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

lib/librte_vhost/vhost.c

@@ -344,6 +344,7 @@ vhost_new_device(void)
 	dev->flags = VIRTIO_DEV_BUILTIN_VIRTIO_NET;
 	dev->slave_req_fd = -1;
 	dev->vdpa_dev_id = -1;
+	dev->postcopy_ufd = -1;
 	rte_spinlock_init(&dev->slave_req_lock);
 
 	return i;