[Libvir] Potential problem on NUMA machines

In the virNodeInfo structure, it's assumed that all nodes of multiprocessor systems have the same number of sockets. But on NUMA machines, it's not mandatory that all SMP nodes have the same number of sockets. So there is a potential problem when computing the number of processors by using the informations returned in the virNodeInfo structure. This problem exists also in Xen (in the "xm info" command) but both Xen and libvirt are wrong.