qemu: Close the agent connection only on agent channel events
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

libvirt/libvirt,taget/libvirt,olafhering/libvirt,rlaager/libvirt,eskultety/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,agx/libvirt,zippy2/libvirt,eskultety/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,olafhering/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,libvirt/libvirt,crobinso/libvirt,datto/libvirt,nertpinx/libvirt,rlaager/libvirt,jardasgit/libvirt,jardasgit/libvirt,rlaager/libvirt,datto/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,zippy2/libvirt,eskultety/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,agx/libvirt,libvirt/libvirt,rlaager/libvirt,jardasgit/libvirt,zippy2/libvirt,crobinso/libvirt,olafhering/libvirt,olafhering/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,datto/libvirt,nertpinx/libvirt,jfehlig/libvirt,taget/libvirt,libvirt/libvirt,VenkatDatta/libvirt,zippy2/libvirt,nertpinx/libvirt,andreabolognani/libvirt,taget/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,agx/libvirt,nertpinx/libvirt,taget/libvirt,eskultety/libvirt,nertpinx/libvirt,datto/libvirt,agx/libvirt,andreabolognani/libvirt,taget/libvirt,eskultety/libvirt,crobinso/libvirt,jardasgit/libvirt,agx/libvirt,crobinso/libvirt,rlaager/libvirt,datto/libvirt