monitor: Throttle event VSERPORT_CHANGE separately by "id"

monitor: Throttle event VSERPORT_CHANGE separately by "id"

VSERPORT_CHANGE is emitted when the guest opens or closes a
virtio-serial port.  The event's member "id" identifies the port.

When several events arrive quickly, throttling drops all but the last
of them.  Because of that, a QMP client must assume that *any* port
may have changed state when it receives a VSERPORT_CHANGE event and
throttling may have happened.

Make the event more useful by throttling it for each port separately.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <1444921716[email protected]>