Make it more obvious about the expected return values.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20170825105913.4060-7-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>