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

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 <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Reviewed-by: liguang <[email protected]>
Acked-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>