qmp: add DEVICE_TRAY_MOVED event It's emitted whenever the tray is moved by the guest or by HMP/QMP commands. Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com> Reviewed-by: Markus Armbruster <armbru@redhat.com> Acked-by: Kevin Wolf <kwolf@redhat.com>