Comment # 3 on bug 1043432 from Fabian Vogt

(In reply to H. Zeng from comment #2)
> Sorry but I don't how to check the "battery and brightness" while the guest
> is in full-screen mode because the host screen is fully covered by the guest
> screen.

You should be able to switch the virtual desktop by escaping the keyboard lock
(using the host key) and hovering over the HUD, pressing Ctrl-F2.

> The status of options in "battery and brightness" including the "Enable
> Power Management" are the same before entering and after exiting the
> full-screen state. I'm not sure whether this is what you are asking.

This is expected, otherwise it would never lock at all ;-)