Libvirt uses an side files to store the internal state of managed domains in order to re-read the state upon libvirtd service restart. However, if the state was saved in inconsistent state, the state wasn't re-read and the corresponding domain was lost.
The domain may disappear.
When the libvirtd service is saving the internal state of a domain, the consistent internal state is saved and domains which may break it are disallowed to be started up.
The domain is not longer frogotten.