Refactor code to handle multiple protocols at the same time

[guillaumerose]

Currently, the code is designed to handle one protocol at a time. A good refactoring would be to allow many different hypervisors to talk to the same gvproxy instance.

For instance, UML and Qemu or Hyperkit and Qemu VMs talking together on the same virtual switch.

It is not probably something useful in the real world but it can help for testing.