Virtual Desktop Host Pool API should allow the name which is longer than 24 characters

After tested with azure rest api, I found [Virtual Desktop Host Pool API](https://github.com/Azure/azure-rest-api-specs/blob/7c8fd7f0705552f77bf442c567c854af01ab4805/specification/desktopvirtualization/resource-manager/Microsoft.DesktopVirtualization/preview/2019-12-10-preview/desktopvirtualization.json#L1076) doesn't allow the name which is longer than 24 characters. But Azure Portal allows to create this resource with name which is longer than 24 characters after tested on Azure Portal. So I assume it's the swagger property validation issue.

Error message:
```
desktopvirtualization.HostPoolsClient#CreateOrUpdate: Invalid input: autorest/validation: validation failed: parameter=hostPoolName constraint=MaxLength value="wvdpool-pooled-dev-s-eastus" details: value length must be less than or equal to 24
```

After checked azure-rest-api-spec, seems there is a [length validation](https://github.com/Azure/azure-rest-api-specs/blob/7c8fd7f0705552f77bf442c567c854af01ab4805/specification/desktopvirtualization/resource-manager/Microsoft.DesktopVirtualization/preview/2019-12-10-preview/desktopvirtualization.json#L2957) on host pool name. Maybe it's the root cause.