[UDisks2] Correctly demarshall QDbusObjectPath in `isEncryptedCleartext`
DBus ObjectPaths can not be converted by QVariant::toString()
, but have
to be explicitly converted. toString
always returns an empty QString().
This fixes isEncryptedCleartest always returning false, even for
encrypted partitions. As isEncryptedCleartext()
is currently unused
this bug is not noticable.