On starting the guixsd-usb-install-0.13.0.x86_64-linux image, the
documentation says:

qemu-system-x86_64 -m 1024 -smp 1 \
  -net user -net nic,model=virtio -boot menu=on \
  -drive file=guixsd.img \
  -drive file=guixsd-usb-install-0.13.0.system

I'm using qemu 2.0.0 on Ubuntu 14.04.5.

At some point during the boot process the console got garbled with snow.

I found out the passing -vga std to qemu fixes that in my case.

Cheers,
Eddy