vmm/amd64: Bhyve should keep guest CPUID updated when using explicit CPUID settings

[rosenfeld_grumpf.hope-2000.org]

Summary

This is a port of the following bhyve improvement from illumos:

16647 bhyve should keep guest CPUID updated when using explicit CPUID settings
https://www.illumos.org/issues/16647

Some bits of information returned by the CPUID instruction depend on the
values system software previously set in the CPU's control registers.
These include

• Leaf 1 ecx: bit 27 changes depending on whether XSAVE support is enabled in cr4
• Leaf 1 edx: bit 9 changes depending on whether the APIC is enabled in MSR 0x1B
• Leaf D subleaf 0/subleaf 1 ebx: values change depending on the bits set in xcr0

bhyve should ensure that VMs with explicitly-provided CPUID values
account for the guest's current CR/MSR values before returning the
explicitly-set values for these leaves. (The legacy CPUID handler,
legacy_emulate_cpuid, already handles this for VMs where explicit
CPUID settings aren't in use.)

[rosenfeld_grumpf.hope-2000.org]

Rebase.

[rosenfeld_grumpf.hope-2000.org]

Rebase.

[wanpengqian_gmail.com]

One data point from running guests on the legacy path, in case it helps reviewing this: the leaf 0xD subleaf 0 EBX dependency on XCR0 is not cosmetic. We once returned a cached host value there (sized for the host's XCR0, not the guest's), and Linux 7.0 guests (Ubuntu 26.04) disabled XSAVE as inconsistent and then panicked; recomputing EBX from the guest's current XCR0 on every CPUID fixed it. So having the explicit-CPUID path do the same as legacy_emulate_cpuid() looks right to us.