Add UFFD snapshot pager

[sjmiller609]

Summary

• recreates the UFFD pager work on top of the new Firecracker resume branch
• folds in the pager cache concurrency optimization from Optimize UFFD pager cache concurrency #259
• supersedes Add Firecracker UFFD snapshot pager #257 and Optimize UFFD pager cache concurrency #259; old branches are preserved for comparison

Tests

• git diff --check origin/main...hypeship/uffd-pager-v2
• go test ./lib/hypervisor/firecracker -count=1
• go test ./lib/instances -count=1 could not run in this checkout because embedded VMM and guest-agent binaries are not present