conf: add infrastructure for tolerating certain post parse callback failures
    
    Some failures of the post parse callback can be tolerated. This is
    specifically desired when loading the configs of existing VMs. In such
    case the post parse callback should not really be modifying anything
    in the definition.
    
    This patch adds a parse flag VIR_DOMAIN_DEF_PARSE_ALLOW_POST_PARSE_FAIL
    which will allow the callbacks to report non-fatal failures by returning
    a positive return value. In such case the field 'postParseFailed' in the
    domain definition is set to true, to notify the drivers that the
    callback failed and possibly needs to be re-run.