cpus: fix TCG kick timer leak

This is an alternative fix to Marc-André's original patch.
Reported-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Suggested-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Message-Id: <20180927171724.30128-1-alex.bennee@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

cpus.c

@@ -983,6 +983,8 @@ static void start_tcg_kick_timer(void)
     if (!tcg_kick_vcpu_timer && CPU_NEXT(first_cpu)) {
         tcg_kick_vcpu_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
                                            kick_tcg_thread, NULL);
+    }
+    if (tcg_kick_vcpu_timer && !timer_pending(tcg_kick_vcpu_timer)) {
         timer_mod(tcg_kick_vcpu_timer, qemu_tcg_next_kick());
     }
 }
@@ -990,9 +992,8 @@ static void start_tcg_kick_timer(void)
 static void stop_tcg_kick_timer(void)
 {
     assert(!mttcg_enabled);
-    if (tcg_kick_vcpu_timer) {
+    if (tcg_kick_vcpu_timer && timer_pending(tcg_kick_vcpu_timer)) {
         timer_del(tcg_kick_vcpu_timer);
-        tcg_kick_vcpu_timer = NULL;
     }
 }