`azurerm_virtual_machine` should ForceNew
Fixes#6873
```
make testacc TEST=./builtin/providers/azurerm TESTARGS='-run=TestAccAzureRMVirtualMachine_ChangeAvailbilitySet'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
TF_ACC=1 go test ./builtin/providers/azurerm -v
-run=TestAccAzureRMVirtualMachine_ChangeAvailbilitySet -timeout 120m
=== RUN TestAccAzureRMVirtualMachine_ChangeAvailbilitySet
--- PASS: TestAccAzureRMVirtualMachine_ChangeAvailbilitySet (976.35s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/azurerm
976.367s
```
390e1acb3c