kvm: Remove unreachable code in kvm_dirty_ring_reaper_thread()

The code at the tail end of the loop in kvm_dirty_ring_reaper_thread()
is unreachable, because there is no way for execution to leave the
loop. Replace it with a g_assert_not_reached().

(The code has always been unreachable, right from the start
when the function was added in commit b4420f198dd8.)

Resolves: Coverity CID 1547687
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20240815131206.3231819-3-peter.maydell@linaro.org

accel/kvm/kvm-all.c

@@ -1525,11 +1525,7 @@ static void *kvm_dirty_ring_reaper_thread(void *data)
         r->reaper_iteration++;
     }
 
-    trace_kvm_dirty_ring_reaper("exit");
-
-    rcu_unregister_thread();
-
-    return NULL;
+    g_assert_not_reached();
 }
 
 static void kvm_dirty_ring_reaper_init(KVMState *s)