Libguestfs - Jun 2021 - [libguestfs/nbdkit] Rust example ramdisk, writes fail with "Function not implemented" (#13)

I tried this and it works fine for me:

```
$ nbdfuse /tmp/ram --command nbdkit -v -s
./plugins/rust/target/release/examples/libramdisk.so
...
$ dd if=/dev/urandom of=/tmp/ram/nbd bs=512 count=1
1+0 records in
1+0 records out
512 bytes copied, 0.000274812 s, 1.9 MB/s
$ fusermount3 -u /tmp/ram
```

What version of nbdkit, nbdfuse, etc are you using?  On what platform?
Did you try it without the log filter?