Considering that many of the most talented Solaris hackers have left Oracle and now mostly work at other companies that depend on illumos, I strongly doubt that they can't write their own device drivers.

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?