virtio-serial: report frontend connection state via monitor

Libvirt wants to know about the guest-side connection state of some
virtio-serial ports (in particular the one(s) assigned to guest agent(s)).
Report such states with a new monitor event.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1080376
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
