vmm: Move defines from x86.c to x86/bhyve.h

[cperciva]

Summary

The values CPUID_BHYVE_FEATURES and CPUID_BHYVE_FEAT_EXT_DEST_ID are
useful for guests, not just hosts; so they belong in a header file in
sys/x86/include rather than simply in the .c file implementing the
bhyve host side.

The original addition of these defines took place without adding a
copyright statement, but since I'm moving them into a new file I've
added the original author's standard copyright (Amazon).

MFC after: 2 weeks
Fixes: 313a68ea20b4 ("bhyve: Add CPUID_BHYVE_FEATURES leaf")

[cperciva]

Adjusted include guard to _X86_BHYVE_H_ instead of _BHYVE_H_

[kib]

Perhaps the defines should go into x86/include/specialreg.h together with all other CPUID bits definitions.

[cperciva]

In D55430#1268961, @kib wrote:

Perhaps the defines should go into x86/include/specialreg.h together with all other CPUID bits definitions.

I considered that, but specialreg.h only has CPUID bits relating to hardware; hypervisor-related CPUID bits have always gone into hypervisor-specific header files.

[kib]

IMO other bits should be also moved to specialreg.h