network: disable tx csum offload on vif interfaces

It doesn't work with HVMs - more precisely with (ancient) qemu in
stubdomain.

network/vif-route-qubes

@@ -63,5 +63,7 @@ fi
 log debug "Successful vif-route-qubes $command for $vif."
 if [ "$command" = "online" ]
 then
+  # disable tx checksumming offload, apparently it doesn't work with our ancient qemu in stubdom
+  do_without_error ethtool -K $vif tx off
   success
 fi