Tomorrow they (Oracle) will decide to "slash the costs, do restructuring and refocus on their core businesses" as Cisco recently did, and guess what? ^_^
Joyent probably has fairly homogeneous racks of servers. They only need a few drivers for their entire system, and they have people capable of writing them. If they can port KVM from Linux to illumos, what makes you think they can't port or write a driver?