KVM: selftests: Fix vmxon_pa == vmcs12_pa == -1ull nVMX testcase for !eVMCS

KVM: selftests: Fix vmxon_pa == vmcs12_pa == -1ull nVMX testcase for !eVMCS

The "vmxon_pa == vmcs12_pa == -1ull" test happens to work by accident: as
Enlightened VMCS is always supported, set_default_vmx_state() adds
'KVM_STATE_NESTED_EVMCS' to 'flags' and the following branch of
vmx_set_nested_state() is executed:

        if ((kvm_state->flags & KVM_STATE_NESTED_EVMCS) &&
            (!guest_can_use(vcpu, X86_FEATURE_VMX) ||
             !vmx->nested.enlightened_vmcs_enabled))
                        return -EINVAL;

as 'enlightened_vmcs_enabled' is false. In fact, "vmxon_pa == vmcs12_pa ==
-1ull" is a valid state when not tainted by wrong flags so the test should
aim for this branch:

        if (kvm_state->hdr.vmx.vmxon_pa == INVALID_GPA)
                return 0;

Test all this properly:
- Without KVM_STATE_NESTED_EVMCS in the flags, the expected return value is
'0'.
- With KVM_STATE_NESTED_EVMCS flag (when supported) set, the expected
return value is '-EINVAL' prior to enabling eVMCS and '0' after.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Tested-by: Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20231205103630.1391318-11-vkuznets@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>