zone: track guest-to-physical CPU mappings

[li041]

Summary

• add bidirectional per-zone mappings between guest-visible CPU IDs and physical CPU IDs
• keep CpuSet responsible for physical CPU ownership
• initialize CPU mappings when a zone is created
• assign dense guest CPU IDs according to the physical CPUs listed by the current bitmap-based configuration
• return None for unmapped or out-of-range CPU ID lookups

Motivation

HvZoneConfig::cpus currently describes physical CPUs as a bitmap, while
CpuSet only records physical CPU ownership.

Architecture code that exposes logical CPU IDs to a guest or routes guest CPU
operations needs an explicit translation between guest CPU IDs and physical CPU
IDs. This is especially important for zones assigned sparse physical CPUs. For
example, physical CPUs 1 and 3 are exposed as guest CPUs 0 and 1.

This PR adds the zone-level mapping infrastructure. It does not change interrupt
delivery or CPU startup behavior by itself; architecture-specific consumers can
use these mappings in follow-up changes.