Split virtcontainers into smaller packages

[sameo]

Currently all the virtcontainers code lives under the virtcontainers package, making code documentation not clear and all mixed together. See https://godoc.org/github.com/containers/virtcontainers

We should start splitting the code into different logical packages. For example:

• api for gathering the https://github.com/containers/virtcontainers/blob/master/documentation/api/1.0/api.md
• utils for the externally reachable utilities (things like RunningOnVMM() or SerializeParams()
• agent for adding all agent related structures and interfaces

[sboeuf]

This will certainly add some complexity into the code since we won't be able to access everything as easily as we do today, due to the fact that everything is from the same package main. Seems like a pretty big change for the purpose of clearer documentation.

[egernst]

This issue was moved to kata-containers/runtime#150