hyperv: use x86 generic code to do the hypervisor detection

This is first step to move the generic part of HV code into kernel instead
of module, so that it is possible to use hypercall to implement some other
paravirtualization code in the kernel.

Submitted by: Howard Su <howard0su@gmail.com>
Reviewed by: royger, delphij, adrian
Approved by: adrian (mentor)
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D3072