virtio-serial: delete timer if active during exit

The post_load timer was being freed, but not deleted.  This could cause
problems when the timer is armed, but the device is hot-unplugged before
the callback is executed.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>

hw/virtio-serial-bus.c

@@ -1038,6 +1038,7 @@ void virtio_serial_exit(VirtIODevice *vdev)
     g_free(vser->ports_map);
     if (vser->post_load) {
         g_free(vser->post_load->connected);
+        qemu_del_timer(vser->post_load->timer);
         qemu_free_timer(vser->post_load->timer);
         g_free(vser->post_load);
     }