block/copy-before-write: use file child instead of backing

block/copy-before-write: use file child instead of backing

We are going to publish copy-before-write filter, and there no public
backing-child-based filter in Qemu. No reason to create a precedent, so
let's refactor copy-before-write filter instead.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-Id: <20210824083856[email protected]>
Signed-off-by: Hanna Reitz <[email protected]>