[Libguestfs] [PATCH nbdkit filters-v2 0/5] Introduce filters.

Rebased filters patch. Requires current git master + the locks / thread model fix (https://www.redhat.com/archives/libguestfs/2018-January/msg00128.html) So a few changes here since last time: The "introduce filters" and "implement filters" patches are squashed together. I introduced a concept of .prepare and .finalize. These run before and after the data serving phase respectively. They're not a lot of use for plugins because you can just use .open and .close instead, and therefore I didn't expose them to plugins (only internally in the backend). However for filters they make more sense since they give you a place to prepare and finalize. They're necessary also for filters because filter.open could not call the next open method, save the (plugin) handle, then construct an nbdkit_next_ops object, and call pread. It _has_ to be split into two operations. This patch series is incomplete in at least three ways: - We still need a way for filter.open to call down to the next open function, so it can adjust the readonly flag (for copyonwrite filter, TBD). - Partition filter is obviously not complete. - Need to write a copyonwrite filter. Rich.