Event RTC_CHANGE is "emitted when the guest changes the RTC time" (and
the RTC supports the event).  What if there's more than one RTC?
Which one changed?  New @qom-path identifies it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <87a6ejnm80.fsf@pond.sub.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>