block/copy-before-write: bdrv_cbw_append(): replace child at last

block/copy-before-write: bdrv_cbw_append(): replace child at last

Refactor the function to replace child at last. Thus we don't need to
revert it and code is simplified.

block-copy state initialization being done before replacing the child
doesn't need any drained section.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210824083856.17408-14-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>