docs: document missing VSERPORT_CHANGE event

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 <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Reviewed-by: Wenchao Xia <[email protected]>
Signed-off-by: Luiz Capitulino <[email protected]>