daemon: Autodetect lock driver directory

daemon: Autodetect lock driver directory

When running libvirtd from a build directory, libvirtd would load lock
drivers from system directory unless explicitly overridden by setting
LIBVIRT_LOCK_MANAGER_PLUGIN_DIR environment variable. Since we already
autodetect driver directory if libvirt is build with driver modules, we
can use the same trick to automagically set lock driver directory.