block: Catch backing files assigned to non-COW drivers
Since we parse backing.* options to add a backing file from the command line when the driver didn't assign one, it has been possible to have a backing file for e.g. raw images (it just was never accessed). This is obvious nonsense and should be rejected. Signed-off-by: NKevin Wolf <kwolf@redhat.com> Reviewed-by: NEric Blake <eblake@redhat.com>
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