Mac: unmount should unregister virtualization root in the kext

[sanoursa]

Unmount should send a message to the kext to unregister the virtualization root and do any necessary cleanup as a result. For example, when the final virtualization root on a mount point is unregistered, we could re-enable the auth cache.

Similar logic should kick in when the kext detects that the provider has gone down without a clean handshake.

[pmj]

I've got a prototype implementation for this now, although it currently uses linear search in an array, and the nuts & bolts of the data structure are mixed in with the business logic, so I'm going to split this up before posting the PR.