Add support for hotplug/hotremove of paths to VMs

[stgraber]

Suggested by @brauner

Currently we support adding/removing full disks (block) but not the 9p/virtiofs volumes.

[stgraber]

Looking around in QMP, there's no way for us to do this with 9p right now as there's no way to add an fsdev to a running daemon. It may be possible for virtiofs though as for that we just need chardev and device, both of which are supported in QMP, will need to do some extra tests for that.

This means that at least initially, we'll have to prevent path hotplug if migration.stateful is set to true or if the system is otherwise not supporting virtiofs.

[stgraber]

Done some initial work on this, getting virtiofsd to get spawned and stopped on hotplug/hot-remove is easy enough, adding some restrictions to not needlessly spawn 9p and to check for migration.stateful was pretty simple too.

Now I'm at the stage where I need to fill in the QMP bits and hope that this part works properly too.