vhost-user+postcopy: Use qemu_set_nonblock

Use qemu_set_nonblock rather than a simple fcntl; cleaner
and I have no reason to change other flags.
Reported-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-user.c

@@ -1115,7 +1115,7 @@ static int vhost_user_postcopy_advise(struct vhost_dev *dev, Error **errp)
         error_setg(errp, "%s: Failed to get ufd", __func__);
         return -1;
     }
-    fcntl(ufd, F_SETFL, O_NONBLOCK);
+    qemu_set_nonblock(ufd);
 
     /* register ufd with userfault thread */
     u->postcopy_fd.fd = ufd;