Add kernel-paged differential snapshots with hotplug

[sjmiller609]

summary

• add dirty-page differential snapshots based on Cloud Hypervisor's existing dirty-log infrastructure and upstream PR vmm: add diff snapshots cloud-hypervisor/cloud-hypervisor#8609
• preserve a fixed private file layout across boot and virtio-mem regions so resized VMs can snapshot and restore
• configure tagged Kernel builds to create draft GitHub releases for v*-kernel.*

validation

• cargo test -p vmm --lib --features kvm (89 tests)
• cargo test -p virtio-devices --lib --features kvm (102 tests)
• cargo check -p cloud-hypervisor
• cargo build --locked --all --release --features mshv
• actionlint on the release workflow
• live Chromium lifecycle with 4→8 GiB virtio-mem resize and 4→8 vCPU resize
• differential standby wrote a 484 MB delta, then restored with CDP immediately available
• 25/25 fanout from an 8-vCPU, 8-GiB snapshot: 451 ms average and 534 ms p95 fork-to-CDP

status

This remains experimental. The release is intended for Hypeman compatibility testing before any production rollout.