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: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

block/backup.c

@@ -307,7 +307,7 @@ static void coroutine_fn backup_run(void *opaque)
                                 BACKUP_SECTORS_PER_CLUSTER - i, &n);
                     i += n;
 
-                    if (alloced == 1) {
+                    if (alloced == 1 || n == 0) {
                         break;
                     }
                 }