x86: Add one-off test to verify setting LA57 fails when it's unsupported

Add an i386-only test to check that setting CR4.LA57 fails when 5-level
paging is not exposed to the guest.  Old versions of KVM don't intercept
LA57 by default on VMX, which means a clever guest could set LA57
without it being detected by KVM.

This test is i386-only because toggling CR4.LA57 in long mode is
illegal, i.e. won't verify the desired KVM behavior.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200930043436.29270-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>