The SO major is going to change in the upcoming update of the Hetzner
rescue system, which will cause NixOps to break because it's statically
using the SO major 0 while the new rescue system will have the major
number 1.
I'm still keeping the udevSoMajor attribute to retain backwards-
compatibility with older NixOps versions.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>