There is no way to declare a preferred block size for Loopback

The convenience function `nbd.Loopback` does not yet allow to pass in a preferred block size. It would make sense to allow that - an `*os.File` backend for example, could derive that from `syscall.Stat_t.Blksize`. This should either be part of the `Device` interface, an extra parameter, or a separate struct with options passed to `Loopback`.