nspawn: do not fail if udev is not running

If /sys is read only filesystem, e.g., nspawn is running in container,
then usually udev is not running. In such a case, let's assume that
the interface is already initialized. Also, this makes nspawn refuse
to use the network interface which is under renaming.

Fixes #14223.

(cherry picked from commit 26208d5b)