Run documentation examples with `cargo test`

[andreeaflorescu]

The documentation examples should be run along with the unit tests. Currently, cargo test does not run the examples because cargo does not have support for running doc tests when cross-compiling.
Cargo test issue: rust-lang/cargo#2684

A possible workaround would be to skip the crates that cannot be built with gnu (one of them is x86 as far as I remember because some ioctl have different parameters).
Another workaround is making firecracker buildable with both gnu and musl and run the unit tests with both.

There might be other solutions out there.

[dianpopa]

Fixed by #639 which also added an integration tests for making sure documentation examples are successfully built and exercised.