Azure AEM extension issue

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

## Describe the bug

**Command Name**
`az vm aem set
Extension Name: aem. Version: 0.1.1.`

**Errors:**
```
'str' object has no attribute 'value'
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())
...
cliextensions/aem/azext_aem/custom.py, ln 31, in set_aem
skip_storage_analytics=skip_storage_analytics)
cliextensions/aem/azext_aem/custom.py, ln 57, in __init__
os_type = self._vm.storage_profile.os_disk.os_type.value.lower()
AttributeError: 'str' object has no attribute 'value'
```

## 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 vm aem set -g {} -n {} --subscription {}`

## Expected Behavior

## Environment Summary
```
Linux-4.15.0-1108-azure-x86_64-with-debian-10.2 (Cloud Shell)
Python 3.6.10
Installer: DEB

azure-cli 2.20.0

Extensions:
aem 0.1.1
ai-examples 0.2.5

```
## Additional Context