qemuPrepareNVRAM: Save domain after NVRAM path generation
    
    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: Michal Privoznik <mprivozn@redhat.com>