chardev: comment details for CLOSED event

chardev: comment details for CLOSED event

It was unclear before on what does the CLOSED event mean.  Meanwhile we
add a TODO to fix up the CLOSED event in the future when the in/out
ports are different for a chardev.

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: "Marc-André Lureau" <marcandre.lureau@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180620073223.31964-2-peterx@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>