Notifier will be used for signaling powerdown request to guest in
a more general way and intended to replace very specific
qemu_irq_rise(qemu_system_powerdown) and will allow to remove global
variable qemu_system_powerdown.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>