Make diff snapshots transactional

[JackThomson2]

Store kvm dirty bitmap inside firecrackers internal bitmap so no data is lost incase of an error

Changes

As we load the KVM bitmap store it inside the internal firecracker bitmap. This is done incase of a failure while creating the snapshot, as the data is stored until it's completed successfully.

Reason

Making snapshot transactional

Ticket here: #4545

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license. For more information on following Developer
Certificate of Origin and signing off your commits, please check
CONTRIBUTING.md.

PR Checklist

• If a specific issue led to this PR, this PR closes the issue.
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this
  PR.
• API changes follow the Runbook for Firecracker API changes.
• User-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.
• New TODOs link to an issue.
• Commits meet
  contribution quality standards.

---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

Attention: Patch coverage is 94.54545% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 82.14%. Comparing base (8b15271) to head (88e13c4).

Files	Patch %	Lines
src/vmm/src/vstate/memory.rs	94.54%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4580   +/-   ##
=======================================
  Coverage   82.13%   82.14%           
=======================================
  Files         255      255           
  Lines       31267    31283   +16     
=======================================
+ Hits        25682    25698   +16     
  Misses       5585     5585           

Flag	Coverage Δ
4.14-c5n.metal	79.64% <94.54%> (+0.01%)	⬆️
4.14-c7g.metal	?
4.14-m5n.metal	79.62% <94.54%> (+0.01%)	⬆️
4.14-m6a.metal	78.85% <94.54%> (+0.01%)	⬆️
4.14-m6g.metal	76.70% <90.90%> (+0.01%)	⬆️
4.14-m6i.metal	79.63% <94.54%> (+0.01%)	⬆️
4.14-m7g.metal	76.70% <90.90%> (+0.01%)	⬆️
5.10-c5n.metal	82.16% <94.54%> (+<0.01%)	⬆️
5.10-c7g.metal	?
5.10-m5n.metal	82.15% <94.54%> (+<0.01%)	⬆️
5.10-m6a.metal	81.45% <94.54%> (+0.01%)	⬆️
5.10-m6g.metal	79.47% <90.90%> (+0.01%)	⬆️
5.10-m6i.metal	82.14% <94.54%> (+<0.01%)	⬆️
5.10-m7g.metal	79.47% <90.90%> (+0.01%)	⬆️
6.1-c5n.metal	82.15% <94.54%> (+<0.01%)	⬆️
6.1-c7g.metal	?
6.1-m5n.metal	82.14% <94.54%> (+<0.01%)	⬆️
6.1-m6a.metal	81.45% <94.54%> (+0.01%)	⬆️
6.1-m6g.metal	79.46% <90.90%> (+<0.01%)	⬆️
6.1-m6i.metal	82.14% <94.54%> (+<0.01%)	⬆️
6.1-m7g.metal	79.46% <90.90%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[roypat]

Looks good! I think everything could be squashed into a single commit (at least the formatting changes from f497630 should go into 5c50b59), but other than that I think we can merge this :)

[pb8o]

Yep, looks good to me. As Patrick said maybe squash the last two commits and maybe consider using psutil instead of df, but other than that looks good to me.