CI: Use cargo-llvm-cov for coverage

[mkroening]

Closes #124.
Fixes #198.

This adopts cargo-llvm-cov for our coverage reports, replacing the custom bash scripts.

[codecov]

Codecov Report

Merging #193 (a02f342) into master (b13a88e) will increase coverage by 21.43%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #193       +/-   ##
===========================================
+ Coverage   20.45%   41.89%   +21.43%     
===========================================
  Files          17       16        -1     
  Lines        2659     2454      -205     
===========================================
+ Hits          544     1028      +484     
+ Misses       2115     1426      -689     

Impacted Files	Coverage Δ
src/macros.rs
src/linux/virtio.rs	13.29% <0.00%> (+4.25%)	⬆️
src/linux/uhyve.rs	61.85% <0.00%> (+7.05%)	⬆️
src/vm.rs	78.50% <0.00%> (+25.57%)	⬆️
src/linux/mod.rs	44.51% <0.00%> (+44.51%)	⬆️
src/linux/vcpu.rs	66.40% <0.00%> (+66.40%)	⬆️
src/lib.rs	100.00% <0.00%> (+89.17%)	⬆️

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[mkroening]

This is ready for review now, but requires #199 to be solved for this to work.

[mkroening]

@jounathaen, using rust's bundled lld instead of the system ld works around the issue, so we won't have to wait for new hardware.

[mkroening]

bors r=jounathaen

[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