Create Net devices during configuration instead of boot-time

[acatangiu]

Following the more decoupled design we can now create net devices on their configuration path, rather than saving a config and delaying creation to boot-time.

Such a model allows user-errors (like invalid resources or permissions) to be reported as part of the configuration step rather than later at attempted boot.

It would also remove the time window between configuration and boot where changes to the host system (creating Taps, changing permissions, etc) would cause microVM boot failures (race condition between validation and commitment of resources).

Such a model should also decrease code complexity and should provide better efficiency to the process of configuring and starting a microVM (no more moving configurations around in memory and validating resources both at config time and boot-time).