KVM: selftests: Extend MSRs test to validate vCPUs without supported features

KVM: selftests: Extend MSRs test to validate vCPUs without supported features

Add a third vCPUs to the MSRs test that runs with all features disabled in
the vCPU's CPUID model, to verify that KVM does the right thing with
respect to emulating accesses to MSRs that shouldn't exist.  Use the same
VM to verify that KVM is honoring the vCPU model, e.g. isn't looking at
per-VM state when emulating MSR accesses.

Link: https://lore.kernel.org/r/20250919223258.1604852-48-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>