Well, people are still working on the problem. You can run some runtimes directly on Xen, like the JVM and Haskell.
http://www.eluaproject.net/home/overview
EDIT: I found this on YouTube: https://www.youtube.com/watch?v=W_aXsutL4rQ
I was interested in this a while back, but never took the plunge. Seems you can run it in an 'emulator' on i386.
So eLua and these projects are similar in, say, the same way that a typical RTOS is similar to running Linux on AWS. They both have kernels, but the environments are vastly different.
http://www.artima.com/weblogs/viewpost.jsp?thread=239339
ROS calls itself a 'meta-operating system', I guess the definition of an OS is sort of blurry!