docs: document missing VSERPORT_CHANGE event

The VSERPORT_CHANGE event was added in e2ae6159.  The patch for
this event was prepared at a time when this file was gone, even
though it got applied immediately after dfab4892 restored this
file.  Duplicate the documentation into this file, so that
anyone using this file instead of qapi will not miss out on this
new event.

* docs/qmp/qmp-events.txt (VSERPORT_CHANGE): Add.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Wenchao Xia <wenchaoqemu@gmail.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

docs/qmp/qmp-events.txt

@@ -579,6 +579,22 @@ Example:
                     "host": "127.0.0.1", "sasl_username": "luiz" } },
         "timestamp": { "seconds": 1263475302, "microseconds": 150772 } }
 
+VSERPORT_CHANGE
+---------------
+
+Emitted when the guest opens or closes a virtio-serial port.
+
+Data:
+
+- "id": device identifier of the virtio-serial port (json-string)
+- "open": true if the guest has opened the virtio-serial port (json-bool)
+
+Example:
+
+{ "event": "VSERPORT_CHANGE",
+    "data": { "id": "channel0", "open": true },
+    "timestamp": { "seconds": 1401385907, "microseconds": 422329 } }
+
 WAKEUP
 ------