python-blivet: Fix udev path for Debian Wheezy.

Only affects "useNixUdev = false" and we now include the udev paths for
the latest Debian Wheezy release, so we can properly deploy Hetzner
machines with NixOps again as they've updated their rescue system to
Wheezy.

This fixes NixOS/nixops#137, thanks to @lovek323 for reporting.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>