Rather than let "EthernetInterface" be the base EMAC NetworkInterface,
insert an "EMACInterface" class.
EthernetInterface then derives from EMACInterface and EthInterface.
A Wi-Fi driver can derive from EMACInterface and WiFiInterface - this
will be more logical than deriving from EthernetInterface and
WiFiInterface.
This does mean adding a couple of virtual inheritances to avoid
duplicate NetworkInterfaces:
NetworkInterface
/ \
virtual / \ virtual
/ \
EMACInterface WiFiInterface
\ /
\ /
\ /
MyCustomWiFiInterface