Add SIO, VR, and ZEN

* SIO:- scheduler is based on the deadline scheduler but it's more like a mix between no-op and deadline.In other words, SIO is like a lighter version of deadline but it doesn't do any kind of sorting, so it's aimed mainly for random-access devices (like SSD hard disks) where request sorting is no needed (as any sector can be accesed in a constant time, regardless of its physical location).

* V(R):- The next request is decided based on its distance from the last request, with a multiplicative penalty of `rev_penalty' applied for reversing the head direction. A rev_penalty of 1 means SSTF behaviour. As this variable is increased, the algorithm approaches pure SCAN. Setting rev_penalty to 0 forces SCAN.

* Zen IO scheduler - Primarily based on Noop, deadline, and SIO IO schedulers. FCFS, dispatches are back-inserted, deadlines ensure fairness. Should work best with devices where there is no travel delay.