[libvirt] Is it possible to use more then 2 CPUs in the guest?

Hi, in my server i use an Intel Quad Core CPU and i want to use in one WindowsXP guest all 4 cores. The device manager shows four CPU (Qemu Virtual CPU version 0.10.5). When i start prime95 or superpi, only 2 of them are in use. CPU-Z shows also 2 Cores, instead of 4 like the device manager. How can i use all four cores in the guest? Greetings