While we report the read and written byte stats for every single layer
of the backing chain, qemu in fact reports them only for the frontend.

Split out the relevant stats into a separate function so that we can
later fix this bug and stop reporting it for backing chain entries where
they don't make sense.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>