Fix monitor ref counting when adding event handle

Fix monitor ref counting when adding event handle

When closing a monitor using qemuMonitorClose(), we are aware of
the possibility the monitor is still being used somewhere:

    /* NB: ordinarily one might immediately set mon->watch to -1
     * and mon->fd to -1, but there may be a callback active
     * that is still relying on these fields being valid. So
     * we merely close them, but not clear their values and
     * use this explicit 'closed' flag to track this state */

but since we call virEventAddHandle() on that monitor without increasing
its ref counter, the monitor is still freed which makes possible users
of it quite unhappy. The unhappiness can lead to a hang if qemuMonitorIO
tries to lock mutex which no longer exists.