I thought if you were a private company you can't be bought out unless you want to sell?
I honestly don't know what shareholder currently holds the majority in Valve, but every company, public or unlisted, can be bought out if the potential seller agrees to the offer the buyer is making: it's certainly true that's easier to perform buyouts (even hostile) on public companies, but why should be it not possible to buy out private ones?
I can see Microsoft using very convincing arguments trying to get Valve majority, and make them replace the Linux SteamOS with a Windows SteamOS soon after they succeed.