Implement L2 learning to avoid flooding

[AkihiroSuda]

socket_vmnet should learn the MAC addresses of the VMs, set the MAC addresses to the struct conn objects, and avoid flooding when possible.

https://github.com/lima-vm/socket_vmnet/blob/8b16e519d0e41af20531141972dcb57542c7e337/main.c#L85-L89

https://github.com/lima-vm/socket_vmnet/blob/8b16e519d0e41af20531141972dcb57542c7e337/main.c#L180-L205

https://github.com/lima-vm/socket_vmnet/blob/8b16e519d0e41af20531141972dcb57542c7e337/main.c#L512-L542