Update rust-vmm dependencies

Update rust-vmm dependencies, necessitating these changes:

• VhostUserDaemon::start() now takes a mutable reference to the Listener instead of ownership,
• The exit event EventFd has been replaced by an EventConsumer and EventNotifier pair. We used to store kill_evt as an EventFd to write to in VhostUserFsBackend::drop(), but this should actually be already done by VhostUserHandler::drop() (in vhost-user-backend) ever since vhost commit f6b7e49e[1], so I think it’s time to drop it. VhostUserHandler directly owns our VhostUserFsBackend without further wrapping, so both are dropped at exactly the same time.

Note that this keeps vm-memory on 0.17.1 instead of 0.17.2 or 0.18.0: To update to 0.18.0, we’d need some further changes to other rust-vmm crates, and updating to 0.17.2 creates some conflicts in rust-vmm crates because its re-export of GuestAddressSpace isn’t very useful (its memory type is the new GuestMemory, but it would need to be the old GuestMemory aka now GuestMemoryBackend to be useful). I’ll send a PR for the 0.17.2 issue to vm-memory, but for now just cap the version at 0.17.1.

[1] https://github.com/rust-vmm/vhost/commit/f6b7e49e3f9ea (“handler: send exit event when destructing”)