I had my Qubes without internet for two weeks. It was the message “libxenlight failed to create sys-net”.
I tried everything, without effect. I did not found any solution on the internet. And I remember many questions about it, here.
Finally I created the sys-usb, which I haven’ t, and it started to work, using the usb wifi. But it didn’t last long.
So I decided to delete sys-net and create another one with the command “sudo qubesctl state.sls qvm.sys-net”. And everything returned to normal !!! I am sure it was a error in my old sys-net, but I didin’t find it.