Reboot to play a game is way too much of a hassle (IMO). What I'd want is a hypervisor that could handle graphics acceleration for one virtual machine at a time. When I wanted to play I just switched virtual machine and moved the ability to accelerate graphics from my workstation virtual machine to the gaming virtual machine (similar to the way some notebooks that have an integrated and a more powerful external graphics card can switch between them during runtime).

That would be a dream come true for me (if it worked well).