Implement missing file system operations in virtionfs

[Peter-JanGootzen]

These are all non-essential for experiments:

• [ ] create
  • Send-side programmed but untested, no receive-side
• [ ] setattr
• [ ] flock
• [ ] readdir and readdirplus
• [ ] mkdir
• [ ] rmdir
• [ ] mknod
• [ ] symlink
• [ ] unlink
• [ ] fallocate
• [ ] rename
• [ ] forget and batch_forget