processSerialChangedEvent: Close agent monitor early
    
    https://bugzilla.redhat.com/show_bug.cgi?id=890648
    
    So, imagine you've issued an API that involves guest agent. For
    instance, you want to query guest's IP addresses. So the API acquires
    QUERY_JOB, locks the guest agent and issues the agent command.
    However, for some reason, guest agent replies to initial ping
    correctly, but then crashes tragically while executing real command
    (in this case guest-network-get-interfaces). Since initial ping went
    well, libvirt thinks guest agent is accessible and awaits reply to the
    real command. But it will never come. What will is a monitor event.
    Our handler (processSerialChangedEvent) will try to acquire
    MODIFY_JOB, which will fail obviously because the other thread that's
    executing the API already holds a job. So the event handler exits
    early, and the QUERY_JOB is never released nor ended.
    
    The way how to solve this is to put flag somewhere in the monitor
    internals. The flag is called @running and agent commands are issued
    iff the flag is set. The flag itself is set when we connect to the
    agent socket. And unset whenever we see DISCONNECT event from the
    agent. Moreover, we must wake up all the threads waiting for the
    agent. This is done by signalizing the condition they're waiting on.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>