[libvirt] New APIs for checking some object properties (v2)

This is a second posting of the proposal from earlier today http://www.redhat.com/archives/libvir-list/2009-October/msg00556.html The changes since last time - Renamed some existing internal APIs which had bad names which clashed with our new public APis. - Fix the comment bugs Daniel mentioned - Change the return status from '> 0' to '1', so we can explicitly add other positive integer return codes if ever required - Implement the new APIs for (nearly) all drivers Daniel