SAN FRANCISCO — Virtual reality is everywhere you look at the Game Developers Conference in San Francisco, but few people are talking about its most serious limitation: How do you get real-world motion, such as walking or running, to translate into a game without running the risk of a player sprinting headlong into a wall? Its Omni device looks kind of like a treadmill, but instead of having moving parts, the Omni uses a slippery concave surface to let players walk, sidestep and run in place through virtual worlds. Available for preorder on Virtuixs website, the Omni costs $499 and is compatible with any PC game.