[DataPlane] LoopBack interface

[GeorgyKirichenko]

DataPlane offers a Virtio-based virtual interface to provide communication between the host system and the network. Processing packets here does not involve anything other than passing them to the interface's output queue.
However, in some cases we would like to process locally running packages in the same pipeline as external ones. A good example here is balancer probes - this mechanic will allow us to get rid of any additional utilities such as nfq/checktun/etc.