Honor that string properties with EmbeddedInstance qualifier are
handled as Instances, also for Override

This mapping of string->instance happens _after_ Override features are
checked for compatibility. If one does the mapping before, konkrete
fails with "EmbeddedInstance qualifier allowed only on string
properties".

This fixes https://github.com/rnovacek/konkretcmpi/issues/3
Merge pull request #4 from kkaempf/override_of_embedded_instances_as_string

Honor that string properties with EmbeddedInstance qualifier are