I found the reason. ;-)
Below code fixed my problem. Setting extra properties of GuestCustomizationSection fixed it. Look at red font color section.
GuestCustomizationSectionType
guest_settings = vm.GetGuestCustomizationSection();
guest_settings.ComputerName = computerName;
guest_settings.ChangeSid =
true;
guest_settings.ChangeSidSpecified =
true;
guest_settings.UseOrgSettings =
false;
guest_settings.UseOrgSettingsSpecified =
true;
guest_settings.DomainName =
"<your doamin name>";
guest_settings.DomainUserName =
"<user name>";
guest_settings.DomainUserPassword =
"<password>";
guest_settings.JoinDomainEnabled =
true;
guest_settings.JoinDomainEnabledSpecified =
true;
task = vm.UpdateSection(guest_settings);
task.WaitForTask(timeout);