[libvirt] Can not shutdown and reboot KVM virtual machine

Hi, I found I can not shutdown and reboot VMs in my KVM box. It seems I can shutdown a VM which has guest OS installed, but can not shutdown the VM which has no guest OS installed. Reboot can not work with this error message: this function is not supported by the hypervisor: virDomainReboot Any ideas? Thanks! I am using libvirt-0.6.4 and KVM-86. Regards, Qian