[libvirt] Redefining the XML of VM

Hi, I'm a newbie to Libvirt, i want to know if i can change the vm definition xml dynamically. like change cpu, network value. I found the hook in Libvirt, and i can get the xml from standard input. but i change it and out put to standard output, it no effect. I was try to execute “virsh define” shell command in python script on hook, but it was blocked and unabled to execute another Libvirt command. Is there create vm hook in Libvirt? Any help is appreciated. Regards,