S390 - fix for bnc719881. Only L2 capable devices can be enslaved.

The bug was caused by the fact that all "Unconfigured" interfaces
were automatically included into list of devices available for
enslaving before checking any other condition.

Bugfix contais refactoring of generating list of devices for enslaving.
Mainly it separates the list creation from UI. Also it introduces
several "getters" and other helpers, so it influences an other
code a bit too.
Updated testsuite.
Updated changelog.