fix: rename page_size_kib to page_size

[acsmyth]

Changes

• update page_size_kib name to the more accurate page_size, since it's actually in bytes, not KiB.
  ...

We leave the old page_size_kib field in there, but mark it as deprecated to avoid any Firecracker code from accidentally using it (and update everything on the Firecracker side to be called page_size instead of page_size_kib). This way we're backward compatible (and can remove the wrongly named field in 2.0), but have correctly named variables in the code base at least.
...

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• I have read and understand CONTRIBUTING.md.
• 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
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---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.14%. Comparing base (70d6e7f) to head (8715c43).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4948   +/-   ##
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  Coverage   83.14%   83.14%           
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  Files         248      248           
  Lines       26923    26925    +2     
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+ Hits        22386    22388    +2     
  Misses       4537     4537           

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[roypat]

Hi @acsmyth,
thanks for catching this! The team's discussed this, and we agreed that although huge pages support is somewhat new, this would still be a breaking change, and so there should be some sort of transition period for users of Firecracker. So for that reason, I think we should just carry two fields for a while (e.g. both page_size and page_size_kib with the broken semantics). That being said, I think most of your PR can go through unchanged, we'll only need to readd page_size_kib to GuestRegionUffdMapping and set it to the same value as page_size in create_guest_memory. We'd also need to add something to DEPRECATED.md warning that we'll remove page_size_kib in an upcoming version.