azure: improve docs and validation

Closes #5186

builder/azure/arm/config.go

@@ -534,6 +534,10 @@ func assertRequiredParametersSet(c *Config, errs *packer.MultiError) {
 		errs = packer.MultiErrorAppend(errs, fmt.Errorf("Specify either a VHD (image_url), Image Reference (image_publisher, image_offer, image_sku) or a Managed Disk (custom_managed_disk_image_name, custom_managed_disk_resource_group_name"))
 	}
 
+	if isImageUrl && c.ManagedImageResourceGroupName != "" {
+		errs = packer.MultiErrorAppend(errs, fmt.Errorf("A managed image must be created from a managed image, it cannot be created from a VHD."))
+	}
+
 	if c.ImageUrl == "" && c.CustomManagedImageName == "" {
 		if c.ImagePublisher == "" {
 			errs = packer.MultiErrorAppend(errs, fmt.Errorf("An image_publisher must be specified"))

builder/azure/arm/config_test.go

@@ -792,6 +792,26 @@ func TestConfigShouldRejectVhdAndManagedImageOutput(t *testing.T) {
 	}
 }
 
+// If the user specified a build of a VHD, but started with a managed image it should be rejected.
+func TestConfigShouldRejectManagedImageSourceAndVhdOutput(t *testing.T) {
+	config := map[string]interface{}{
+		"image_url":                         "ignore",
+		"location":                          "ignore",
+		"subscription_id":                   "ignore",
+		"communicator":                      "none",
+		"managed_image_resource_group_name": "ignore",
+		"managed_image_name":                "ignore",
+
+		// Does not matter for this test case, just pick one.
+		"os_type": constants.Target_Linux,
+	}
+
+	_, _, err := newConfig(config, getPackerConfiguration())
+	if err == nil {
+		t.Fatal("expected config to reject VHD and Managed Image build")
+	}
+}
+
 func TestConfigShouldRejectCustomAndPlatformManagedImageBuild(t *testing.T) {
 	config := map[string]interface{}{
 		"custom_managed_image_resource_group_name": "ignore",

website/source/docs/builders/azure.html.md

@@ -50,7 +50,10 @@ builder.
     
 #### VHD or Managed Image
 
-The Azure builder can create either a VHD, or a managed image. When creating a VHD the following two options are required.
+The Azure builder can create either a VHD, or a managed image. If you
+are creating a VHD, you **must** start with a VHD.  Likewise, if you
+want to create a managed image you **must** start with a managed
+image.  When creating a VHD the following two options are required.
 
 -   `capture_container_name` (string) Destination container name. Essentially the "directory" where your VHD will be 
     organized in Azure.  The captured VHD's URL will be https://<storage_account>.blob.core.windows.net/system/Microsoft.Compute/Images/<capture_container_name>/<capture_name_prefix>.xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.vhd.