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

[Daniel P. Berrange]

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