KVM: sefltests: Add x86-64 test to verify MMU reacts to CPUID updates

KVM: sefltests: Add x86-64 test to verify MMU reacts to CPUID updates

Add an x86-only test to verify that x86's MMU reacts to CPUID updates
that impact the MMU.  KVM has had multiple bugs where it fails to
reconfigure the MMU after the guest's vCPU model changes.

Sadly, this test is effectively limited to shadow paging because the
hardware page walk handler doesn't support software disabling of GBPAGES
support, and KVM doesn't manually walk the GVA->GPA on faults for
performance reasons (doing so would large defeat the benefits of TDP).

Don't require !TDP for the tests as there is still value in running the
tests with TDP, even though the tests will fail (barring KVM hacks).
E.g. KVM should not completely explode if MAXPHYADDR results in KVM using
4-level vs. 5-level paging for the guest.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-20-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>