refactor: cleanup vmm::snapshot module

[roypat]

Implement the suggestions from #4523 to give a cleaner API for the
snapshot module. Conceptually, the change is that we now store the thing
we are snapshotting inside of the Snapshot struct.
Implementation-wise, this allows us to deserialize the snapshot data in
a single pass (instead of having to read it twice, once for
deserialization and once for CRC validation), which means we do not have
to first query the snapshot file for the size of the snapshot.

Closes #4691, #4523, #5195

License Acceptance

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CONTRIBUTING.md.

PR Checklist

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkbuild --all to verify that the PR passes
  build checks on all supported architectures.
• I have run tools/devtool checkstyle to verify that the PR passes the
  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the
  Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or
  integration tests.
• I have linked an issue to every new TODO.

---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.32%. Comparing base (4e29a70) to head (1be6b70).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/snapshot-editor/src/info.rs	0.00%	9 Missing ⚠️
src/snapshot-editor/src/utils.rs	0.00%	5 Missing ⚠️
src/snapshot-editor/src/edit_vmstate.rs	25.00%	3 Missing ⚠️
src/vmm/src/snapshot/mod.rs	92.68%	3 Missing ⚠️
src/firecracker/src/main.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5355      +/-   ##
==========================================
+ Coverage   82.30%   82.32%   +0.01%     
==========================================
  Files         265      265              
  Lines       30640    30580      -60     
==========================================
- Hits        25219    25174      -45     
+ Misses       5421     5406      -15     

Flag	Coverage Δ
5.10-c5n.metal	82.29% <69.35%> (-0.04%)	⬇️
5.10-m5n.metal	82.29% <69.35%> (-0.04%)	⬇️
5.10-m6a.metal	81.56% <69.35%> (-0.04%)	⬇️
5.10-m6g.metal	78.87% <66.66%> (-0.05%)	⬇️
5.10-m6i.metal	82.28% <69.35%> (-0.03%)	⬇️
5.10-m7a.metal-48xl	81.55% <69.35%> (?)
5.10-m7g.metal	78.87% <66.66%> (-0.05%)	⬇️
5.10-m7i.metal-24xl	82.25% <69.35%> (?)
5.10-m7i.metal-48xl	82.25% <69.35%> (?)
5.10-m8g.metal-24xl	78.87% <66.66%> (?)
5.10-m8g.metal-48xl	78.87% <66.66%> (?)
6.1-c5n.metal	82.33% <69.35%> (-0.04%)	⬇️
6.1-m5n.metal	82.33% <69.35%> (-0.04%)	⬇️
6.1-m6a.metal	81.60% <69.35%> (-0.05%)	⬇️
6.1-m6g.metal	78.86% <66.66%> (-0.06%)	⬇️
6.1-m6i.metal	82.32% <69.35%> (-0.03%)	⬇️
6.1-m7a.metal-48xl	81.59% <69.35%> (?)
6.1-m7g.metal	78.87% <66.66%> (-0.05%)	⬇️
6.1-m7i.metal-24xl	82.33% <69.35%> (?)
6.1-m7i.metal-48xl	82.33% <69.35%> (?)
6.1-m8g.metal-24xl	78.87% <66.66%> (?)
6.1-m8g.metal-48xl	78.87% <66.66%> (?)

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