Add madt_cpu callback to AcpiDeviceIfClass and use
it for generating LAPIC MADT entries for CPUs.

Later it will be used for generating x2APIC
entries in case of more than 255 CPUs and also
would be reused by ARM target when ACPI CPU hotplug
is introduced there.
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>