Use concrete vCPU structs instead of trait objects

[mkroening]

In #164, I implement additional traits for UhyveCPU on linux, but not on macos.

To be able to use the trait impls, this makes the vCPU structs concrete instead of using trait objects.

[codecov]

Codecov Report

Merging #186 (63fa348) into master (095579f) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #186   +/-   ##
=======================================
  Coverage   30.76%   30.76%           
=======================================
  Files          16       16           
  Lines        4018     4018           
=======================================
  Hits         1236     1236           
  Misses       2782     2782           

Impacted Files	Coverage Δ
src/linux/mod.rs	0.00% <ø> (ø)
src/linux/uhyve.rs	53.77% <0.00%> (ø)
src/linux/virtio.rs	9.13% <0.00%> (ø)
src/vm.rs	28.35% <ø> (ø)

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[jounathaen]

Nice!
bors r+

[bors]

Build succeeded:

• Build (macos-latest)
• Build (ubuntu-latest)
• Cargo tests (macos-latest)
• Cargo tests (ubuntu-latest)
• Clippy
• Format
• KVM Tests
• Rusty-Hermit Kernel based integration tests