Thank the old gods and the new, you can't. Can you imagine how many Pandora's boxes that would open if you could control the host from a guest?
You didn't tell us what your end goal is, but I'm thinking you're running a long task in the guest, after which you want the host to go to sleep/hibernate. Well, you could do that by having a "file" in a shared folder and check its existence from the host. Something like a "I'm done" flag.
Once your guest finishes whatever it's doing, you create the file.
You run a program periodically on your host which checks for that file.
Once the file is there, you delete it and you put the host to sleep.
EDIT: Plus what Martin said, but I wouldn't trust my guest (especially WinXP) to have "the keys" (credentials) to my host.
PS. You could upgrade your VirtualBox, your version is quite old...
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