This appears to be come sort of compatibility issue with Virtualbox 5.0.18. I was able to boot the ubuntu/xenial64 image on Virtualbox 5.0.16. It failed with an identical error after upgrading to 5.0.18, and worked again after downgrading back to 5.0.16.
This appears to be come sort of compatibility issue with Virtualbox 5.0.18. I was able to boot the ubuntu/xenial64 image on Virtualbox 5.0.16. It failed with an identical error after upgrading to 5.0.18, and worked again after downgrading back to 5.0.16.