mibuild/sim: create server.py and server_tb (Proof of concept OK with flterm)

mibuild/sim: create server.py and server_tb (Proof of concept OK with flterm)
Using a server allow us to create a virtual UART (and ethernet TAP in the future).

1) start the server
2) start flterm on the virtual serial port created by the server
3) run the simulation

This will enable us to do serialboot and netboot in simulation.
This will also enable prototyping  ethernet for ARTIQ in simulation.