Comment 6 for bug 971621

Just getting to this now.

Hmm, this is quite awkward.
There are 3 cases to consider for correctly cleaning up the LXC mount dirs

1. Multiple LXC instances (the orig bug)
virt/driver.py stores a global object tracking the mount,
and so will always clean the last LXC instance created.

Now you could let the operating system maintain state and just
`umount /the/lxc/instance/mount/dir`.
Unfortunately a simple umount wont suffice because
depending on the mount method we may need to
`fuesrmount -u` or cleaup nbd devices etc.
So we'd need to store some state as to the
type of mount we performed.
Now maybe things could be arranged so that `umount ...` always works,
though that seems invasive and hacky at first glance.

2. Now because these are long lived mounts, nova could be restarted,
requiring state info for the umount to be maintained outside of nova.
We'd have to be careful to create this atomically, i.e. as a transaction
around the mount operation.

3. If the system is restarted, then the mounts would be too,
and so the persisted mount information should be ignored.