[CI] Code Coverage

[vifino]

Thanks to 659af66 and 329cb4d, we have source-based instrument coverage!

You can see it tracked on codecov.
Coveralls required special stuff for multiple coverage uploads and that seemed too much of a hassle.

There are a couple of issues left until the coverage is gonna get anywhere near 100%:

• Some files are definitely used, but have no executing code, hence coverage shows way too low:
  • src/lib.rs (just mod statements)
  • src/state/mod.rs (mod and pub use)
  • src/errors.rs (derive macro)
• Enum variable usage seem to be incorrectly tracked.
  • src/bin/nvmet/port.rs
  • src/bin/nvmet/namespace.rs
  • src/bin/nvmet/subsystem.rs
• Lots of sysfs error cases are practically impossible to test.
  • At ~70% now, probably not gonna get much better without cheating.
  • Due to defensive coding, a bunch of error cases will never happen unless the kernel is drunk.

I estimate we can get to 70-80% without doing anything about the issues above and just adding more test cases for the actual code.