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: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

block/qcow.c

@@ -596,7 +596,7 @@ static int qcow_co_writev(BlockDriverState *bs, int64_t sector_num,
     if (qiov->niov > 1) {
         qemu_vfree(orig_buf);
     }
-    free(cluster_data);
+    g_free(cluster_data);
 
     return ret;
 }