On a domain startup, the variable store path is generated if needed.
The path is intended to be generated only once. However, the updated
domain definition is not saved into config dir rather than state XML
only. So later, whenever the domain is destroyed and the daemon is
restarted, the generated path is forgotten and the file may be left
behind on virDomainUndefine() call.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>