target-i386: Introduce x86_cpu_apic_id_from_index() function

This function will be used by both the CPU initialization code and the
fw_cfg table initialization code.

Later this function will be updated to generate APIC IDs according to
the CPU topology.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>