lxc: explicitly error on interface type=ethernet

It isn't implemented and does not work:

error: internal error: guest failed to start: /usr/lib/libvirt/libvirt_lxc: option '--veth' requires an argument
syntax: /usr/lib/libvirt/libvirt_lxc [OPTIONS] ...

We previously threw an explicit error, but this changed in
22cff52a , which I suspect was
untested for LXC

src/lxc/lxc_process.c

@@ -559,8 +559,6 @@ static int virLXCProcessSetupInterfaces(virConnectPtr conn,
             break;
 
         case VIR_DOMAIN_NET_TYPE_ETHERNET:
-            break;
-
         case VIR_DOMAIN_NET_TYPE_USER:
         case VIR_DOMAIN_NET_TYPE_VHOSTUSER:
         case VIR_DOMAIN_NET_TYPE_SERVER: