Add UFFD snapshot pager

[sjmiller609]

Summary

• adds a config-gated Firecracker UFFD memory backend for snapshot restore
• starts a per-restore UFFD pager session backed by the snapshot memory file and an optional shared page cache
• applies resume-network mailbox updates through UFFD overlay pages so restore does not mutate the backing memory file
• shards the pager cache and tracks pager timing counters for page-fault, lookup, backing-read, and copy latency

Tests

• go test ./lib/hypervisor/firecracker ./lib/uffdpager ./lib/mailbox ./lib/guest ./lib/system/guest_agent ./lib/oapi -count=1