[migration] Skip virtual rtypes when adding relation definition through add_cube Virtual rtypes should be skipped as they are added dynamically on loading schema. Those are skipped by e.g. schema serialisation or add_entity_type, do the same thing here.

[migration] Avoid unnecessary intermediary commit when migrating a schema Those are low-hanging fruit following changes in ac74476d686c (Fix addition of entity type including boundary constraints on its own attributes): this is no more necesary to commit the get a new relation type definition in the schema. At some point we could/should probably do more on this topic to avoid intermediary commit on e.g. entity type addition, but this requires more work.