chardev/char-win-stdio.c: restore old console mode

chardev/char-win-stdio.c: restore old console mode

If I use `-serial stdio` on Windows, after QEMU exits, the terminal
could not handle arrow keys and tab any more. Because stdio backend
on Windows sets console mode to virtual terminal input when starts,
but does not restore the old mode when finalize.

This small patch saves the old console mode and set it back.

Signed-off-by: Ziming Song <s.ziming@hotmail.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <ME3P282MB25488BE7C39BF0C35CD0DA5D8CA82@ME3P282MB2548.AUSP282.PROD.OUTLOOK.COM>