qemu: Close the agent connection only on agent channel events
processSerialChangedEvent processes events for all channels. Commit
2af51483 broke all agent interaction if a channel other than the agent
closes since it did not check that the event actually originated from
the guest agent channel.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1236924
Fixes up: https://bugzilla.redhat.com/show_bug.cgi?id=890648