cpg: Avoid list corruption

Move the list_del calls from message_handler_req_lib_cpg_finalize() to
cpg_lib_exit_fn().

finalize() is called on a different thread to the init() and exit() functions
and the list is not multithread protected, so it can become corrupted causing
corosync to crash.

rhbz#1025321

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>