[libvirt] [PATCHv2 2/2] qemu: eliminate "Ignoring open failure"

These patches are a 2nd go at eliminating the warning message described in: https://bugzilla.redhat.com/show_bug.cgi?id=624447 The first round of patches was here: https://www.redhat.com/archives/libvir-list/2012-January/msg00503.html This time I've addressed Eric's concerns from the first round, but also taken the chance to refactor virFileOpenAs() to remove some historical baggage, make it easier to understand, and also more useful as a general purpose replacement for open(2). Note that, due to the way the diff interleaves the new and old code, the diffs of the virFileOpenAs() don't make a lot of sense - it's much simpler to apply the patches and look at the new function as a whole. (Or maybe install my favorite newly found program of the week, "meld", and "git-meld": http://meldmerge.org/ https://github.com/wmanley/git-meld I think this is the nicest looking visual diff tool I've ever seen.)