Hyper-V: Code refactor to avoid redundancy of MSR values on x86 and arm64

Refactor the code to put split the MSR values for x86 and arm64
Hyper-V. Code not yet built. This is one of several patches for
the arm64 Hyper-V enablement.

Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D37103