[Libguestfs] [PATCH nbdkit] Add new filter for rate-limiting connections.

For virt-v2v we have been discussing how to limit network bandwidth. The initial discussion has been around how to use cgroups to do this limiting, and that is still probably what we will go with in the end. However this patch gives us another possibility for certain virt-v2v inputs, especially VDDK. We could apply a filter on top of the nbdkit plugin which limits the rate at which it copies data. For example, to limit the rate to 1 Mbps (megabit per second) we could now do: nbdkit --filter=rate vddk [etc] rate=1M The filter is implemented using a simple Token Bucket (https://en.wikipedia.org/wiki/Token_bucket) and is quite simple while at the same time using the fully parallel thread model. Rich.