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: NEric Blake <eblake@redhat.com> Reviewed-by: NPeter Krempa <pkrempa@redhat.com> Reviewed-by: NJán Tomko <jtomko@redhat.com> Message-Id: <20200706203954.341758-10-eblake@redhat.com> Signed-off-by: NKevin Wolf <kwolf@redhat.com>
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