[Libguestfs] [PATCH libnbd] copy: Set POSIX_FADV_SEQUENTIAL and POSIX_FADV_DONTNEED

I'm trying to emulate what we do in nbdkit-file-plugin to avoid the page cache getting trashed when copying to/from local files: https://gitlab.com/nbdkit/nbdkit/-/commit/aa5a2183a6d16afd919174b5ba8222b... This patch works well when reading from a local file as you can see from the results in the commit message. However when writing to a local file it has absolutely no effect, and I've no idea why. (Even checked with strace and it's making the correct fadvise64 system calls). For the reading case it has a slight performance impact which could be problematic. Should we only enable it with a flag? I would prefer to fix it so it's always beneficial, but it's not too easy to do that without adding more threads and synchronization. Rich.