vmm/amd64: support CPUID functions 0x40000100 and above

[rosenfeld_grumpf.hope-2000.org]

Summary

CPUID functions 0x40000100 and above mirror the original contents of
the hypervisor CPUID functions at 0x40000000. This allows a guest to
query the real hypervisor signature and features when the the default
hypervisor functions at 0x40000000 have been overridden to identify
as something else, such as KVM or HyperV.

[wanpengqian_gmail.com]

+1 from a user of this layout: keelOS does exactly this for its Hyper-V enlightenments (0x40000000-0x4000000A become Hyper-V's, bhyve's own leaves move to 0x40000100/0x40000101). It is the convention KVM uses as well (KVM_CPUID_SIGNATURE_NEXT = 0x40000100), and a FreeBSD guest's identify_hypervisor_cpuid_base() already walks the bases in 0x100 steps, so it finds bhyve either way. Works for us with FreeBSD, Linux and Windows guests.

[chuck]

LGTM, but are there other places in the kernel that should now know about the new offset?

[chuck]

I done goofed and accepted this as bhyve. Please wait for one of the others to weigh in as well.

[rosenfeld_grumpf.hope-2000.org]

Rebase and update commit message.