Support for Data Disk Attachments on VM's with VM Extensions installed

[tombuildsstuff]

This PR fixes an issue where the Virtual Machine Extension object cannot be posted back to the Virtual Machine API - which causes an issue when using the `azurerm_virtual_machine_data_disk_attachment resource on a Virtual Machine with a VM Extension configured.

Before this fix:

$ acctests azurerm TestAccAzureRMVirtualMachineDataDiskAttachment_virtualMachineExtension
=== RUN   TestAccAzureRMVirtualMachineDataDiskAttachment_virtualMachineExtension
--- FAIL: TestAccAzureRMVirtualMachineDataDiskAttachment_virtualMachineExtension (584.25s)
	testing.go:513: Step 1 error: Error applying: 1 error(s) occurred:

		* azurerm_virtual_machine_data_disk_attachment.test: 1 error(s) occurred:

		* azurerm_virtual_machine_data_disk_attachment.test: Error updating Virtual Machine "acctestvm1479430537935323945" (Resource Group "acctestrg1479430537935323945") with Disk "acctest1479430537935323945": compute.VirtualMachinesClient#CreateOrUpdate: Failure sending request: StatusCode=0 -- Original Error: autorest/azure: Service returned an error. Status=400 Code="InvalidRequestContent" Message="The request content was invalid and could not be deserialized: 'Could not find member 'resources' on object of type 'ResourceDefinition'. Path 'resources', line 1, position 1655.'."
FAIL
FAIL	github.com/terraform-providers/terraform-provider-azurerm/azurerm	584.613s

After this fix:

$ acctests azurerm TestAccAzureRMVirtualMachineDataDiskAttachment_virtualMachineExtension
=== RUN   TestAccAzureRMVirtualMachineDataDiskAttachment_virtualMachineExtension
--- PASS: TestAccAzureRMVirtualMachineDataDiskAttachment_virtualMachineExtension (653.67s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	654.047s

Fixes #1600

[katbyte]

LGTM 🚀

[tombuildsstuff]

Virtual Machine * tests pass:

[jamespatetz]

Can you add a version tag to this? Thanks

[ghost]

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