qmp: emit the WAKEUP event when the guest is put to run

qmp: emit the WAKEUP event when the guest is put to run

Today, the WAKEUP event is emitted when a wakeup _request_ is made.
This could be the system_wakeup command, for example.

A better semantic would be to emit the event when the guest is
already running, as that's what matters in the end. This commit does
that change.

In theory, this could break compatibility. In practice, it shouldn't
happen though, as clients shouldn't rely on timing characteristics of
the events. That is, a client relying that the guest is not running
when the event arrives may break if the event arrives after the guest
is already running.

This commit also adds the missing documentation for the WAKEUP event.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>