While reseting ROOT or any user using Azure CLI on VM

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

## Describe the bug

**Command Name**
`az vm user update`

**Errors:**
```
'str' object has no attribute 'value'
Traceback (most recent call last):
python3.6/site-packages/knack/cli.py, ln 215, in invoke
cmd_result = self.invocation.execute(args)
cli/core/commands/__init__.py, ln 654, in execute
raise ex
cli/core/commands/__init__.py, ln 718, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
cli/core/commands/__init__.py, ln 711, in _run_job
six.reraise(*sys.exc_info())
lib/python3.6/site-packages/six.py, ln 703, in reraise
raise value
...
azure/cli/core/__init__.py, ln 784, in default_command_handler
return op(**command_args)
cli/command_modules/vm/custom.py, ln 2217, in set_user
if _is_linux_os(vm):
cli/command_modules/vm/custom.py, ln 175, in _is_linux_os
os_type = vm.storage_profile.os_disk.os_type.value if vm.storage_profile.os_disk.os_type else None
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 user update --resource-group {} --name {} --username {} --password {}`

## Expected Behavior

## Environment Summary
```
Linux-5.3.18-lp152.87-default-x86_64-with-glibc2.3.4
Python 3.6.12
Installer: PIP

azure-cli 2.14.2 *

```
## Additional Context