devfs - Change make_dev_covering to use dev_ops

devfs - Change make_dev_covering to use dev_ops

* Change make_dev_covering and all other related functions to use a
  backing dev_ops instead of a backing cdev.
  This allows for a more generic use of make_dev_covering, without the
  need of an explicit backing cdev.

* Also change make_autoclone_dev to use the new make_dev_covering to
  avoid maj/min collision between base clonable devices.