[Libguestfs] [PATCH nbdkit filters-v3 0/7] Introduce filters.

This is still tentative and needs a lot of work, but: - partition filter works, supporting MBR & GPT - prepare and finalize methods fixed - open method can now be changed (allowing readonly flag to be modified) - thread_model can be limited I believe I made most of the changes which were previously suggested in email. I think the only one I didn't was preventing inclusion of both <nbdkit-plugin.h> and <nbdkit-filter.h> in the same file. Rich.