fix: enable clippy as_conversions lint across workspace

[arianbeh]

Changes

• Enabled the clippy::as_conversions lint at the workspace level by updating
  Cargo.toml.
• Resolved all resulting as_conversions violations across the workspace by:
  • Replacing unsafe pointer casts with std::ptr::from_ref / std::ptr::from_mut
    where appropriate (e.g. in cpu-template-helper).
  • Replacing silent integer casts with explicit conversions such as
    i64::from(), u64::try_from(), and usize::try_from().
  • Using localized #[allow(clippy::as_conversions)] in low-level or const
    contexts where as is intentional and required (e.g., enum-to-byte
    encoding in ACPI AML, seccompiler const fns, and libc constant conversions).
  • Cleaning up comparisons and length/size conversions to avoid silent
    truncation.
• Updated the necessary code paths in:
  • cpu-template-helper
  • utils::time
  • acpi-tables (AML, DSDT, MADT, XSDT)
  • seccompiler (bindings, types, lib)
  • rebase-snap
  • jailer (resource_limits)
  • snapshot-editor
  • firecracker (main and examples)
  • vmm (library, benches, and tests)

Reason

This PR implements the final step of #3161, consolidating several
cast-related Clippy lints into as_conversions and fixing all violations.

Enabling as_conversions ensures:

• Integer and pointer casts are either:
  • Performed via explicit, safe conversion APIs, or
  • Clearly scoped behind #[allow] where they are intentional and unavoidable.
• Silent truncation, wraparound, and lossy conversions are avoided.
• The workspace stays clean under RUSTFLAGS="-Dwarnings".
• Future code introducing unsafe as casts will be flagged consistently.

All changes preserve existing behavior while making the code safer and more
explicit about type boundaries.

Verification

Locally ran:

• RUSTFLAGS="-Dwarnings" cargo clippy --all --all-targets
• cargo fmt

All packages build clean with no Clippy warnings.

I was not able to run tools/devtool checkbuild or tools/devtool checkstyle
because /dev/kvm is not available in my environment.

Closes #3161.