git: devfs - Change make_dev_covering to use dev_ops

[Simon 'corecode' Schubert]

Alex Hornung wrote:
    * Also change make_autoclone_dev to use the new make_dev_covering to
      avoid maj/min collision between base clonable devices.
Does that mean that the entire minor range is now available for the 
cloned devs?

cheers
  simon