char: report frontend open/closed state in 'query-chardev'

char: report frontend open/closed state in 'query-chardev'

In addition to the on-line reporting added in the previous patch, allow
libvirt to query frontend state independently of events.

Libvirt's path to identify the guest agent channel it cares about differs
between the event added in the previous patch and the QMP response field
added here. The event identifies the frontend device, by "id". The
'query-chardev' QMP command identifies the backend device (again by "id").
The association is under libvirt's control.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1080376

Reviewed-by: Amit Shah <[email protected]>
Signed-off-by: Laszlo Ersek <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Luiz Capitulino <[email protected]>