qemu-timer: introduce timer_deinit

In some cases, a timer was set to NULL so that we could check if it is
initialized.  Use the timer_list field instead, and add a timer_deinit
function that NULLs it.

It then makes sense that timer_del be a no-op (instead of a crasher) on
such a de-initialized timer.  It avoids the need to poke at the timerlist
field to check if the timers are initialized.

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

include/qemu/timer.h

@@ -594,6 +594,17 @@ static inline QEMUTimer *timer_new_ms(QEMUClockType type, QEMUTimerCB *cb,
     return timer_new(type, SCALE_MS, cb, opaque);
 }
 
+/**
+ * timer_deinit:
+ * @ts: the timer to be de-initialised
+ *
+ * Deassociate the timer from any timerlist.  You should
+ * call timer_del before.  After this call, any further
+ * timer_del call cannot cause dangling pointer accesses
+ * even if the previously used timerlist is freed.
+ */
+void timer_deinit(QEMUTimer *ts);
+
 /**
  * timer_free:
  * @ts: the timer