Win32: Make PPSSPP aware of the Windows Vista/7/8/8.1 Virtual Store.

I had intended on committing this in my last pull request regarding portable mode, but I ended up forgetting. This'll make it so when PPSSPP is run from a read only directory(e.g. program files), it'll really fail when trying to create files and not use the Virtual Store UAC provides(http://stackoverflow.com/questions/11830864/how-can-i-make-that-when-i-run-my-application-thorugh-the-vs-it-will-prompt-for, http://stackoverflow.com/questions/4730053/how-to-disable-virtualstore-for-c-programs, except we use asinvoker, since we *don't* want admin access), so the fallbacks work correctly.