COLO-compare: Add new parameter to communicate with remote colo-frame

We add the "notify_dev=chardevID" parameter. After that colo-compare can connect with
remote(currently just for Xen, KVM-COLO didn't need it.) colo-frame through chardev socket,
it can notify remote(Xen) colo-frame to handle checkpoint event.

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>