rcu: actually register threads that have RCU read-side critical sections

Otherwise, grace periods are detected too early!

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/rcu.c

@@ -216,6 +216,8 @@ static void *call_rcu_thread(void *opaque)
 {
     struct rcu_head *node;
 
+    rcu_register_thread();
+
     for (;;) {
         int tries = 0;
         int n = atomic_read(&rcu_call_count);