multipass mount /mnt instance:/etc results in nonresponsive instance

**Describe the bug**
multipass mount /mnt instance:/etc results in nonresponsive instance

**To Reproduce**
1. `multipass mount /mnt ubuntu:/etc`

Instance is non-responsive after this point.

**Expected behavior**
solutions:
- multipass warns
- it is possible to stop the instance using `multipass stop`
- it is possible to umount without ssh and restart sshd

**Workaround**

`multipass umount ubuntu:/etc`
`systemctl restart snap.multipass.multipassd.service`

**Logs**
```
Oct 03 19:16:36 machine multipassd[2068]: mounting /home/user/ubuntu => /etc in ubuntu
Oct 03 19:16:36 machine multipassd[2068]: process program '/snap/multipass/2604/bin/sshfs_server'
Oct 03 19:16:36 machine multipassd[2068]: process arguments '10.182.122.22, 22, ubuntu, /home/user/ubuntu, /etc, 1000:-1,, 1000:-1,'
Oct 03 19:16:36 machine multipassd[2068]: starting: /snap/multipass/2604/bin/sshfs_server 10.182.122.22 22 ubuntu /home/user/ubuntu /etc 1000:-1, 1000:-1,
Oct 03 19:16:52 machine multipassd[2068]: Cannot open ssh session on "ubuntu" shutdown: ssh connection failed: 'Socket error: Connection reset by peer'
Oct 03 19:16:52 machine multipassd[2068]: Stopping mount '/etc' in instance "ubuntu"
Oct 03 19:16:52 machine multipassd[2068]: QMP: {"return": {}}
{"timestamp": {"seconds": 1601745412, "microseconds": 52347}, "event": "POWERDOWN"}
Oct 03 19:17:22 machine multipassd[2068]: QProcess: Destroyed while process ("/snap/multipass/2604/bin/sshfs_server") is still running.
Oct 03 19:17:22 machine multipassd[2068]: Mount '/etc' in instance "ubuntu" has stopped
```

**Additional info**
- OS: ArchLinux
- 1.4.0
- `multipass info --all` is not responsive: `info failed: ssh connection failed: 'Timeout connecting to ...`

**Additional context**
Probably related to issue #17

Reproduce `multipass mount /mnt ubuntu:/etc` and inspect the mount handling in `multipassd` and `sshfs_server`, using the provided logs and the related issue #17 as starting points. Done means the command warns or remains recoverable, and `multipass stop` and `multipass umount` work without SSH access or restarting the service.

Written by the indexing model from the issue text.