[Virtio-fs] [virtiofsd] MR merged: vhost_user_backend: Remove nested RwLock

Implement `VhostUserBackend` instead of `VhostUserBackendMut`, in
order to avoid having a nested `RwLock`.  When using
`VhostUserBackendMut`, it needs to be wrapped in an `RwLock` (or
`Mutex`) which doesn't make sense, because our implementation uses
its own `Mutex` for synchronization.

This MR contains a 2nd commit that replaces the internal `Mutex` by an `RwLock`.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/91