nixos/virtualisation.hypervGuest: use elevator=noop

Microsoft recommends the NOOP I/O scheduler for disk performance in HYPER-V:

https://docs.microsoft.com/en-us/windows-server/virtualization/hyper-v/best-practices-for-running-linux-on-hyper-v

> NOOP is a first-in first-out queue that passes the schedule decision
> to be made by the hypervisor. It is recommended to use NOOP as the
> scheduler when running Linux virtual machine on Hyper-V.