Convert free to g_free to match g_new and g_malloc functions.

Fixes: cc2b33ea ("softmmu/dirtylimit: Implement vCPU dirtyrate calculation periodically")
Fixes: baa60983 ("softmmu/dirtylimit: Implement virtual CPU throttle")
Signed-off-by: alloc.young <alloc.young@outlook.com>
Reviewed-by: Hyman Huang <yong.huang@smartx.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <SA1PR11MB67604ECD85AFF34BEB3072F7F5E3A@SA1PR11MB6760.namprd11.prod.outlook.com>
Signed-off-by: Hyman Huang <yong.huang@smartx.com>