VirtualCPU: Take descriptive arguments

[mkroening]

This PR moves the transmutes inside many VirtualCPU functions outside, moving the responsibility for a valid reference to the caller.

[codecov]

Codecov Report

Merging #294 (baea893) into master (3be9a3b) will decrease coverage by 0.04%.
The diff coverage is 65.00%.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
- Coverage   63.44%   63.40%   -0.05%     
==========================================
  Files          16       16              
  Lines        2380     2391      +11     
==========================================
+ Hits         1510     1516       +6     
- Misses        870      875       +5     

Impacted Files	Coverage Δ
src/linux/vcpu.rs	69.85% <64.51%> (-0.50%)	⬇️
src/vm.rs	76.47% <66.66%> (+0.29%)	⬆️

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[stlankes]

bors r+

[bors]

Build succeeded:

• Check (aarch64, macos-latest)
• Check (x86_64, macos-latest)
• Check (x86_64, ubuntu-latest)
• Clippy (aarch64, macos-latest)
• Clippy test-kernels
• Clippy (x86_64, macos-latest)
• Clippy (x86_64, ubuntu-latest)
• Coverage
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• rusty_demo
• Test (macos-latest)
• Test (ubuntu-latest)