selftests: kvm: vmx_dirty_log_test: skip the test when VMX is not supported

vmx_dirty_log_test fails on AMD and this is no surprise as it is VMX
specific. Bail early when nested VMX is unsupported.

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