qapi: move RTC_CHANGE to the target schema

qapi: move RTC_CHANGE to the target schema

A few targets don't emit RTC_CHANGE, we could restrict the event to
the tagets that do emit it.

Note: There is a lot more of events & commands that we could restrict
to capable targets, with the cost of some additional complexity, but
the benefit of added correctness and better introspection.

Signed-off-by: Marc-André Lureau <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20190214152251[email protected]>