If the native backend (where one is supported) reports a failure in its init() function, fall back to create a backend of the threadpool variant. This required changing the hierarchy a bit so they can co-exist - without turning this into a fully plugin-ized class - as well as making the thread and future features mandatory for the async-io feature, so there is an actual fallback guaranteed to be available. Task-number: QTBUG-136763 Pick-to: 6.11 Change-Id: Ib87c42bb67eec446278be2e43ca76f594155ab84 Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>