block: Tweak error message related to qemu-img amend

When converting a 1.1 image down to 0.10, qemu-iotests 060 forces
a contrived failure where allocating a cluster used to replace a
zero cluster reads unaligned data.  Since it is a zero cluster
rather than a data cluster being converted, changing the error
message to match our earlier change in 'qcow2: Make distinction
between zero cluster types obvious' is worthwhile.

Suggested-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 20170508171302.17805-1-eblake@redhat.com
[mreitz: Commit message fixes]
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/qcow2-cluster.c

@@ -1797,7 +1797,8 @@ static int expand_zero_clusters_in_l1(BlockDriverState *bs, uint64_t *l1_table,
             }
 
             if (offset_into_cluster(s, offset)) {
-                qcow2_signal_corruption(bs, true, -1, -1, "Data cluster offset "
+                qcow2_signal_corruption(bs, true, -1, -1,
+                                        "Cluster allocation offset "
                                         "%#" PRIx64 " unaligned (L2 offset: %#"
                                         PRIx64 ", L2 index: %#x)", offset,
                                         l2_offset, j);

tests/qemu-iotests/060.out

@@ -143,7 +143,7 @@ read failed: Input/output error
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-qcow2: Marking image as corrupt: Data cluster offset 0x52a00 unaligned (L2 offset: 0x40000, L2 index: 0); further corruption events will be suppressed
+qcow2: Marking image as corrupt: Cluster allocation offset 0x52a00 unaligned (L2 offset: 0x40000, L2 index: 0); further corruption events will be suppressed
 qemu-img: Error while amending options: Input/output error
 
 === Testing unaligned reftable entry ===