* src/xen/xen_driver.c: Add support for VIR_MIGRATE_PERSIST_DEST flag
* src/xen/xend_internal.c: Add support for VIR_MIGRATE_UNDEFINE_SOURCE flag
* include/libvirt/virterror.h, src/util/virterror.c: Add new errorcode
  VIR_ERR_MIGRATE_PERSIST_FAILED