QDBusReply allows one to extract a QVariant and the type reply from an
error reply and getting a default-constructed value. This is useful when
a valid reply can never contain the default-constructed value (0, false,
empty strings, empty arrays, etc.), so it simplifies error checking.
More importantly, qdbusxml2cpp was changed a while ago from generating
QDBusReply to generating QDBusPendingReply, so we need to have the same
behavior.
Task-number: QTBUG-29046
Change-Id: Ia873b9fd4311c0d4e94f0ef623ba405c20bc0e8c
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>