Take incremental (diff) Firecracker snapshots and rebase them onto a base

Documented steps

1. Enable dirty page tracking at machine-config time: curl --unix-socket /tmp/firecracker.socket -X PUT http://localhost/machine-config -d '{"vcpu_count":2,"mem_size_mib":1024,"smt":false,"track_dirty_pages":true}'
2. Pause the VM and create a base full snapshot: PUT /snapshot/create {"snapshot_type":"Full","snapshot_path":"...","mem_file_path":"..."}
3. Resume, run, and for each checkpoint pause then PUT /snapshot/create {"snapshot_type":"Diff","snapshot_path":"...","mem_file_path":"..."} to write only pages dirtied since the last snapshot into a sparse file
4. Merge a diff layer onto the base in creation order with the snapshot-editor tool: snapshot-editor edit-memory rebase --memory-path path/to/base --diff-path path/to/layer
5. Repeat the rebase for each later layer so the base ends up describing the desired memory state, then use the state file from the same call as the last-merged memory layer