fix: report line coverage instead of function coverage

[roypat]

It seems that the raw output of llvm-cov changed at some point, and we ended upcomparing functoin coverage instead of line coverage. Fix this by instead using the new json output, which will hopefully prevent such goofs in the future.

Fixes #170
Signed-off-by: Patrick Roy roypat@amazon.co.uk

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[roypat]

Maybe we could also talk about getting rid about these coverage files altogether and instead using codecov.io at some point? We did that at Firecracker a year ago or so

[stefano-garzarella]

Maybe we could also talk about getting rid about these coverage files altogether and instead using codecov.io at some point? We did that at Firecracker a year ago or so

Yeah, can you open an issue, so if someone has free cycle can look at it.

[roypat]

Maybe we could also talk about getting rid about these coverage files altogether and instead using codecov.io at some point? We did that at Firecracker a year ago or so

Yeah, can you open an issue, so if someone has free cycle can look at it.

I've opened rust-vmm/community#183 :)