During VM teardown, the field AlephFirecrackerVM().guest_api_process._popen
could be None, and the following traceback be raised:

```python
  File "/root/aleph-vm/vm_supervisor/models.py", line 116, in create
    await vm.teardown()
  File "/root/aleph-vm/vm_supervisor/vm/firecracker_microvm.py", line 429, in teardown
    await self.stop_guest_api()
  File "/root/aleph-vm/vm_supervisor/vm/firecracker_microvm.py", line 424, in stop_guest_api
    self.guest_api_process.terminate()
  File "/usr/lib/python3.8/multiprocessing/process.py", line 133, in terminate
    self._popen.terminate()
AttributeError: 'NoneType' object has no attribute 'terminate'
```