[autoform] Use fieldclass from kwargs in guess_field for non-final relations
This follows up on af969080e7e6 in which a "fieldclass" kwarg has been
added in EntityFieldsForm.field_by_name() before calling guess_field().
In this changeset, this value was only used for final relation but the
way non-final relation actually also changed since we also use
guess_field from these relation (previously, the field class specified
through uicfg was instantiated directly in
EntityFieldsForm.field_by_name()). So in the non-final case in
guess_field() we simply try to retrieve "fieldclass" from kwargs and
fall back to RelationField.