Getting boot log diagnostics for VM started in another subscription than the default crashes az cli

### **This is autogenerated. Please review and update as needed.**

## Describe the bug

**Command Name**
`az vm boot-diagnostics get-boot-log`

**Errors:**
```
Request URL: 'https://management.azure.com/subscriptions/e7b891b9-eb95-45be-b9ed-8c21213fc383/resourceGroups/bbiv-nd-rg/providers/Microsoft.Compute/virtualMachines/vm-node1-ins-prod1-2-0-1152?$expand=REDACTED&api-version=REDACTED'; Request method: 'GET'

Request URL: 'https://management.azure.com/subscriptions/e7b891b9-eb95-45be-b9ed-8c21213fc383/resourceGroups/bbiv-nd-rg/providers/Microsoft.Compute/virtualMachines/vm-node1-ins-prod1-2-0-1152/retrieveBootDiagnosticsData?api-version=REDACTED'; Request method: 'POST'

'RetrieveBootDiagnosticsDataResult' object has no attribute 'done'
Traceback (most recent call last):
python3.6/site-packages/knack/cli.py, ln 233, in invoke
cmd_result = self.invocation.execute(args)
cli/core/commands/__init__.py, ln 660, in execute
raise ex
cli/core/commands/__init__.py, ln 723, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
cli/core/commands/__init__.py, ln 716, in _run_job
six.reraise(*sys.exc_info())
...
cli/command_modules/vm/custom.py, ln 1501, in get_boot_log
uris = LongRunningOperation(cmd.cli_ctx)(poller)
cli/core/commands/__init__.py, ln 958, in __call__
while not poller.done():
AttributeError: 'RetrieveBootDiagnosticsDataResult' object has no attribute 'done'
```

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- _Put any pre-requisite steps here..._
- `az group create -l westus -n $RgName --subscription $MySub`
- `az resource list --subscription $MySub | grep {RgName pattern}` - resource group found
- `az resource list | grep {RgName pattern}` - no resource group found
- `az deployment group create --name $depName \
--resource-group $resGroup \
--subscription $subscr \
--template-file $templatef \
--parameters @$paramf \
--verbose`
- `az vm list --resource-group {}` - optional, no VM was found
- `az vm list --resource-group {} --subscription {}` - optional, VM {vm-node1-...} was found
- #(ResourceNotFound) The Resource 'Microsoft.Compute/virtualMachines/vm-node1-ins-prod1-2-0-0812' under resource group '$resGroup' was not found
`az vm show --resource-group {} --subscription {} --name {} --show-details` - see comment above
- `az vm boot-diagnostics get-boot-log --ids {}` -- crashes

## Expected Behavior
az vm boot-diagnostics get-boot-log should return the boot log or a user error

## Environment Summary
```
Linux-4.14.36-x86_64-with-centos-7.2.1511-Core
Python 3.6.8
Installer: RPM

azure-cli 2.20.0

```
## Additional Context