Refactoring: changing nic_devices visilibity, now private.

nic_devices is used in the as helper function of the
public one nics(), having both as public might be
misleading for the user of this module.

Change-Id: Idf790b6d064a71357ff7d56d386db09e4ac29d47
Signed-off-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17274
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>