Split `driver.New()` from `driver.Start()`, move static inizialization in `driver.New()`

[ffromani]

Followup of #171

the idea is to introduce driver.New() which will return a CPUDriver object fully initialized and ready to be registered, and <instance>.Start() which will register the instance CPUDriver in the runtime and start serving. All the blocking (context-requiring) operations must be moved in Start().

This refactoring will prevent all the class of bugs #171 fixed and will lead to a cleaner and more testable codebase.