[libvirt] issues about Migrating with virsh and libvirt functions

Hi everyone i've been trying to migrate a domain from hostA to hostB.But everytime i did it with "virsh --live migrate <Domain> xen+ssh://root@Destination", it failed and told me something like "Error Domain not found:xenUnifiedDomainLookupByName". The same thing happened when i tried to do the same thing with libvirt API like virDomainMigrate & virDomainMigrateToURI. i am confused about this. So can anybody can tell me why?Thanks a lot. BTW,the OS is CentOS 5.4 with libvirt 0.7.7 and Xen 3.4.0.