Resize window in guest qemu


I am using qemu and virt-manager to install a guest machine with sabayon. I followed It is working ok, better than a previous one in virtualbox, but I can’t use it in the whole screen. I installed spice-vdagent in the guest but the screen is still limited to a square.
Is there something else I am missing to do in the guest?, I found some instructions for gentoo but I don’t know how to follow them:, or if really any of that is applicable.
I tried asking here as sabayon’s forum is very small in comparison, thank’s a lot if someone can help me.