cpu: Introduce get_arch_id() method and override it for X86CPU

get_arch_id() adds possibility for generic code to get a guest-visible
CPU ID without accessing CPUArchState.
If derived classes don't override it, it will return cpu_index.

Override it on target-i386 in X86CPU to return the APIC ID.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: liguang <lig.fnst@cn.fujitsu.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>