qcow2: Fix unaligned preallocated truncation

A qcow2 image file's length is not required to have a length that is a
multiple of the cluster size.  However, qcow2_refcount_area() expects an
aligned value for its @start_offset parameter, so we need to round
@old_file_size up to the next cluster boundary.

Reported-by: Ping Li <pingl@redhat.com>
Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1414049
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171009215533.12530-2-mreitz@redhat.com
Cc: qemu-stable@nongnu.org
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/qcow2.c

@@ -3188,6 +3188,7 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset,
                              "Failed to inquire current file length");
             return old_file_size;
         }
+        old_file_size = ROUND_UP(old_file_size, s->cluster_size);
 
         nb_new_data_clusters = DIV_ROUND_UP(offset - old_length,
                                             s->cluster_size);