If you have qemu installed (or in the SDK), you can start the image with:
  cd path/to/image
  ./flatcar_production_iso.sh -display curses

If you need to use a different ssh key or different ssh port:
  ./flatcar_production_iso.sh -a ~/.ssh/authorized_keys -p 2223 -- -display curses

If you rather you can use the -nographic option instad of '-display curses'. In this
mode you can switch from the vm to the qemu monitor console with: Ctrl-a c
See the qemu man page for more details on the monitor console.

SSH into that host with:
  ssh 127.0.0.1 -p 2222