block/backup: Fix hang for unaligned image size
When doing a block backup of an image with an unaligned size (with respect to the BACKUP_CLUSTER_SIZE), qemu would check the allocation status of sectors after the end of the image. bdrv_is_allocated() returns a result that is valid for 0 sectors in this case, so the backup job ran into an endless loop. Stop looping when seeing a result valid for 0 sectors, we're at EOF then. The test case looks somewhat unrelated at first sight because I originally tried to reproduce a different suspected bug that turned out to not exist. Still a good test case and it accidentally found this one. Signed-off-by: NKevin Wolf <kwolf@redhat.com> Reviewed-by: NEric Blake <eblake@redhat.com>
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