[libvirt] [PATCHv2 0/4] Fix return value of several functions

These patches started out as a more "correct" fix for the patch https://www.redhat.com/archives/libvir-list/2010-July/msg00257.html (and they supercede that patch). Daniel Berrange suggested that it would be better to switch the function virFileOperation over to use the preferred convention of returning -errno on failure, rather than errno. While I was doing that, I also changed virDirCreate for consistency. During this process, I found one bug (in patch 4/4) and made a slight improvement to another function's return so that virFileOperation's return would always be -errno (rather than sometimes being a simple -1). (patch 3/4)