Before the fix https://github.com/openshift/machine-api-provider-azure/pull/182 
When creating a Machine/MachineSet on Azure Stack Hub with diagnostics.boot.storageAccountType: AzureManaged, the Machine creation succeed, but in fact, it doesn't take affect.

After the fix https://github.com/openshift/machine-api-provider-azure/pull/182  When creating a Machine/MachineSet on Azure Stack Hub with diagnostics.boot.storageAccountType: AzureManaged, the Machine creation fails with a 400 error from Azure API, but there is no validation at the Machine API level to prevent this invalid configuration.

4.22

Always 

    1. On an Azure Stack Hub cluster, create a MachineSet with the following configuration:

          diagnostics:
            boot:
              storageAccountType: AzureManaged

    2. Check machine status
    3.
    

Machine create failed with error:
        RESPONSE 400: 400 Bad Request
        ERROR CODE: InvalidParameter
        --------------------------------------------------------------------------------
        {
          "error": {
            "code": "InvalidParameter",
            "message": "Required parameter 'bootDiagnostics.storageAccountUri' is missing (null).",
            "target": "bootDiagnostics.storageAccountUri"
          }
        }

Azure Stack Hub does not support Azure-managed boot diagnostics storage.
So Machine/MachineSet creation should fail with validation error like "Error: Invalid Machine configuration: AzureManaged boot diagnostics is not supported on Azure Stack Hub"