[Virtio-fs] [virtiofsd] MR opened: mount_fd: Use RAII for MountFd reference counting

[virtiofs-bot at sinrega.org]

This commit moves `MountFd` and `MountFds` into a separate crate
and introduces a new type `MountFdRef` that is a strong reference
to a `MountFd`.  `MountFdRef` uses RAII in order to correctly
mutate the refcount of the referenced `MountFd`.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/70