pc: acpi: simplify build_legacy_cpu_hotplug_aml() signature

since IO block used by CPU hotplug is fixed size and
initialized it the same file as build_legacy_cpu_hotplug_aml()
just use ACPI_GPE_PROC_LEN directly instead of passing
it around in several files.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
