qcow2: Fix qcow2_truncate() error return value

If qcow2_alloc_clusters_at() returns an error, we do need to negate it
to get back the positive errno code for error_setg_errno(), but we still
need to return the negative error code.

Fixes: 772d1f973f
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

block/qcow2.c

@@ -3597,7 +3597,7 @@ static int qcow2_truncate(BlockDriverState *bs, int64_t offset,
         if (clusters_allocated < 0) {
             error_setg_errno(errp, -clusters_allocated,
                              "Failed to allocate data clusters");
-            return -clusters_allocated;
+            return clusters_allocated;
         }
 
         assert(clusters_allocated == nb_new_data_clusters);