[libvirt] KVM live migration results in black screen

Hi there, I seem to be able to use either libvirt or virt-manger to live migrate a KVM VM between hosts mounting the same shared storage via NFS. However, when I open the migrated VM on the new host, the VM screen is black and keyboard/mouse has no response. The network is done as well. I wonder if anyone has any experience with this problem. I am using RHEL/CentOS-5. I've tried many times with either Windows or Linux VM. Thank you very much. -- Shi Jin, PhD