block: Add support to warn on backing file change without format

block: Add support to warn on backing file change without format

For now, this is a mechanical addition; all callers pass false. But
the next patch will use it to improve 'qemu-img rebase -u' when
selecting a backing file with no format.

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Peter Krempa <[email protected]>
Reviewed-by: Ján Tomko <[email protected]>
Message-Id: <20200706203954[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>