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

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: 3 weeks
Fixes: 313a68ea20b4 ("bhyve: Add CPUID_BHYVE_FEATURES leaf")
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D55430

(cherry picked from commit 49b6254b3e09ee741f456617111ecb18803459fb)