block/qcow: Fix use of free() instead of g_free()

cppcheck reported this error:

qemu/block/qcow.c:599: error: Mismatching allocation and deallocation: cluster_data

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
diff --git a/block/qcow.c b/block/qcow.c
index c8bfecc..eba5a04 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -596,7 +596,7 @@
     if (qiov->niov > 1) {
         qemu_vfree(orig_buf);
     }
-    free(cluster_data);
+    g_free(cluster_data);
 
     return ret;
 }