Absolutely no less than $1600/day. If you're doing reversing work or special-case driver stuff (can't speak to virtualization, but I'd assume similar), at least $2000/day. This sort of work is very niche, which means that if you're established, you can bill quite high up.
I'm pretty sure if you just ask anyone who has ever contributed a driver to the Linux kernel you can find someone who is willing to do "revesing work or special-case driver stuff" for WAY less than $2000/day.
HNOfficeHours
Designed to allow people to offer advice/expertise, or seek people offering advice for a given problem/specialty. Includes scheduling and search. Fun weekend hack from 2.5 years ago. Now deprecated in favor of better alternatives.