Safely close virtualbox machine on host reboot

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http://askubuntu.com – I'm running Windows 7 inside Virtualbox on Ubuntu 11.10. Everything works fine. I'm running it at startup, but I have a problem with rebooting. When I type sudo reboot now the state of the virtual Windows 7 isn't saved. After the reboot the virtualbox starts, but instead of the running Windows I get the Windows' 7 crash boot menu and the windows is booting again. Is there an option that Ubuntu could send some signal to the virtual box to safely close the instance before the host reboot? (HowTos)