Fix cross-kernel snapshot/restore tests

[pb8o]

Changes

fix: ensure snapshot_restore cross kernel test runs

Since PR#3896 this test always succeeded because it did not find any snapshot directories.

Rewrite and fix the test so that

• we see one test case for each directory, instead of one test per CPU template, so it is clear what is being restored.
• switch to creating the microvm through the API rather than a JSON file so that the Microvm/Snapshot classes can account for all the devices.

In addition, remove the combinations that were added while this test was falsely working but never worked:

• x86 snapshot in host 6.1 -> restore in host 5.10
• arm m6g all combinations, m7g host 4.14 and host 5.10

Also add templates that were not being exercised before: T2A, T2CL

Reason

The test was succeeding falsely.

License Acceptance

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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

All modified and coverable lines are covered by tests ✅

Project coverage is 82.10%. Comparing base (8dbfbfd) to head (8e90208).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4632   +/-   ##
=======================================
  Coverage   82.10%   82.10%           
=======================================
  Files         255      255           
  Lines       31280    31280           
=======================================
  Hits        25681    25681           
  Misses       5599     5599           

Flag	Coverage Δ
4.14-c5n.metal	79.59% <ø> (ø)
4.14-c7g.metal	?
4.14-m5n.metal	79.58% <ø> (+<0.01%)	⬆️
4.14-m6a.metal	78.80% <ø> (ø)
4.14-m6g.metal	76.62% <ø> (ø)
4.14-m6i.metal	79.57% <ø> (ø)
4.14-m7g.metal	76.62% <ø> (?)
5.10-c5n.metal	82.11% <ø> (ø)
5.10-c7g.metal	?
5.10-m5n.metal	82.09% <ø> (ø)
5.10-m6a.metal	81.41% <ø> (ø)
5.10-m6g.metal	79.40% <ø> (ø)
5.10-m6i.metal	82.09% <ø> (ø)
5.10-m7g.metal	79.40% <ø> (?)
6.1-c5n.metal	82.11% <ø> (ø)
6.1-m5n.metal	82.09% <ø> (-0.01%)	⬇️
6.1-m6a.metal	81.40% <ø> (-0.01%)	⬇️
6.1-m6g.metal	79.40% <ø> (ø)
6.1-m6i.metal	82.09% <ø> (+<0.01%)	⬆️
6.1-m7g.metal	79.40% <ø> (?)

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[zulinx86]

I really love this PR!! Just for future reference, let me post before/after for "we see one test case for each directory, instead of one test per CPU template, so it is clear what is being restored."

Before:

integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[C3] PASSED [ 33%]
integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[T2] PASSED [ 66%]
integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[T2S] PASSED [100%]

After:

integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[4.14.336_C3_guest_snapshot] PASSED [ 11%]
integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[5.10.217_C3_guest_snapshot] PASSED [ 22%]
integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[5.10.217-no-acpi_C3_guest_snapshot] PASSED [ 33%]
integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[4.14.336_T2_guest_snapshot] PASSED [ 44%]
integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[5.10.217-no-acpi_T2_guest_snapshot] PASSED [ 55%]
integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[5.10.217_T2_guest_snapshot] PASSED [ 66%]
integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[4.14.336_T2S_guest_snapshot] PASSED [ 77%]
integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[5.10.217-no-acpi_T2S_guest_snapshot] PASSED [ 88%]
integration_tests/functional/test_snapshot_restore_cross_kernel.py::test_snap_restore_from_artifacts[5.10.217_T2S_guest_snapshot] PASSED [100%]

[zulinx86]

We might want to split the first commit into two (one for rewrite and another one for removal of some combinations).