vl: fix comment about when parsing cpu definitions

machine->init() was replaced with machine_class->init()
in 958db90cd54823c33345000c995453a8c9b7a005.

Signed-off-by: Wei Jiangang <weijg.fnst@cn.fujitsu.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

vl.c

@@ -4083,7 +4083,7 @@ int main(int argc, char **argv, char **envp)
     /* Init CPU def lists, based on config
      * - Must be called after all the qemu_read_config_file() calls
      * - Must be called before list_cpus()
-     * - Must be called before machine->init()
+     * - Must be called before machine_class->init()
      */
     cpudef_init();