selftests: kvm: fix vmx_set_nested_state_test

vmx_set_nested_state_test is trying to use the KVM_STATE_NESTED_EVMCS without
enabling enlightened VMCS first.  Correct the outcome of the test, and actually
test that it succeeds after the capability is enabled.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>