acpi: add dummy write function for acpi timer

Otherwise the guest can crash qemu (NULL pointer dereference).

https://bugzilla.redhat.com/show_bug.cgi?id=947691

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1368534544-11826-1-git-send-email-kraxel@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
