Is it possible theoretically? Yes. Oracle have templates for Oracle VM to allow you to do this. The use the templates for development and internal training. I know people who have set it up in VirtualBox also.
Will Oracle allow us regular folk to do it? No. You can not legally get hold of the software unless you have a physical Exadata box. Even then, by installing it on a VM you are breaking your license agreement and therefore the law.
The last couple OpenWorlds, people like myself who've never worked on Exadata have asked for access to these templates and been told no. I seriously don't think you will ever get access to this stuff without buying the kit. Why? Although Exadata does have some extra stuff in the storage cells, the vast majority of the code running is standard Oracle RAC, which you can practice on a VM. The big change is managing/patching the infiniband kit and firmware, which you would not be able to replicate in a virtual environment. Added to that, most of the benefits of the produce relate to the combination of hardware which gives good performance on large parallel operations. None of this would be visible to a VM and would likely cloud the issue. You can imagine people saying, "I tried it and it was slower than normal RAC", when in fact they were messing with a VM that had none of the advantages of the real kit.
I'm not denying there are some nice features in the software, but most of the plus points of Exadata come down to the hardware, so using is VM is pretty pointless. When I speak to some of the Oak Table members who use it, they say it is 95% like using regular Oracle RAC and 5% new stuff to learn, most of which relates to the hardware.
I doubt I will ever get to work on a physical Exadata, so I'm not particularly bothered about a VM.