core: remove qemu_service_io

qemu_service_io was mainly an alias to qemu_notify_event,
currently used only by PPC for timer hack, so call
qemu_notify_event directly.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
