Improve documentation and comments on expects/unwraps

[dianpopa]

We need to clarify our use of unwraps and expects throughout the code (i.e. gracefully terminate program execution in case of invariant being violated).

[andreeaflorescu]

One common usecase that we have to consider is dev/kvm not having the proper permissions.

[bbros-dev]

Specifically, the instruction to reset this after rebooting (does it need to be reset after a logout? To be confirmed) needs to go into the Getting Started.

It does not seem to be the case that the setfacl needs to be reset after each invocation of firecracker (see this comment). However, my machine has had enough changes I'm not sure if that is true for a vanilla installation.

[alindima]

Related to https://github.com/firecracker-microvm/firecracker/issues/593