conf: def: Avoid unnecessary allocation of 'perf' events definition
    
    Some code paths already assume that it is allocated since it was always
    allocated by virDomainPerfDefParseXML. Make it member of virDomainDef
    directly so that we don't have to allocate it all the time.
    
    This fixes crash when attempting to connect to an existing process via
    virDomainQemuAttach since we would not allocate it in that code path.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1350688