Add some comments to document some classes as ABCs

Signed-off-by: Andy Grover <agrover@redhat.com>
model alias follow-up

Don't raise an exception if we can't set emulate_model_alias, just
print a diagnostic.

Rename ui_set_model_alias to setup_model_alias. 'setup' seems a little
more accurate since we don't always set model alias, it depends on
the preference.

Rename export_backstore_as_model pref to export_backstore_name_as_model.

Add a configshell default for export_backstore_name_as_model, set to True.

Signed-off-by: Andy Grover <agrover@redhat.com>