Introduce virDomainDirtyRateCalcFlags to get ready for adding mode parameter to qemuDomainStartDirtyRateCalc. Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn> Signed-off-by: Michal Privoznik <mprivozn@redhat.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>