[Virtio-fs] [virtiofsd] MR opened: Fixes failed open mount point when using file handles with chroot

[virtiofs-bot at sinrega.org]

When using a mount point as a shared directory (e.g. bind mount), using `--sandbox=chroot` and `--inode-file-handles=mandatory/prefer`, virtiofsd fails to open the mount point. This is because the shared directory (`path`) and the mount point (`mountprefix`) are the same, so `path.strip_prefix(mountprefix)` returns an empty string.
---
https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/124