[Libguestfs] [PATCH nbdkit v2 0/4] cache: Implement cache-max-size etc.

These are essentially identical to what was previously posted as patches 6/9 through 9/9 here: https://www.redhat.com/archives/libguestfs/2018-December/msg00145.html except that it has been rebased onto the current git master and retested thoroughly. Rich.