[libvirt] question on kvm

hello everyone: during the days I use kvm,I have got some questions I want to know,please read the following questions. 1.When a kvm VM is running, I use 'virsh setvcpus <domain> <count>' to change the cpu numbers of the VM, and when I use 'virsh dumpxml <domain>',there is a change from '<vcpus>count</vcpus>' to '<vcpus current="currentcount">count</vcpus>', just because of this I can't start the VM after I shutdown it. could you help me about this. 2. when I use "virsh setmaxmem <domain> <value>" to change the max memory of the kvm VM, it says "error: Unable to change MaxMemorySize error: this function is not supported by the connection driver: virDomainSetMaxMemory", I want to know if libvirt has bugs or not. 3.It works fine except I reboot the VM after I use "virsh setmem <domain> <value>" to change the kvm VM boot memory, for example, I change the memory 3000000k, but it can not save the value the next time to reboot. Best wishes and Merry Christmas! SunYongGang