nVMX: Add a test to toggle host (L1) CR4.LA57 on VM-Exit

Expand vmx_cr_load_test() to verify that KVM correclty handles toggling
L1's CR4.LA57 on nested VM-Exit.  LA57 can only be toggled in isolation
via VM-Exit, i.e. be the lone bit the causes a MMU role change, because
MOV CR4 #GPs if LA57 is changed while long mode is active.

Test both nested EPT and shadow paging (for L2); the latter will also
toggle KVM's guest_mode bit in the MMU role due to L1 and L2 sharing an
MMU context, but it obviously shouldn't break.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622211124.3698119-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>