Currently if a user wants to run in headless mode they have to edit the
basic.sh script by hand. Rather than making them edit the script by hand,
add support for a HEADLESS environment variable which can be set to 1 to
add necessary command line arguments to qemu.
Currently users have to modify the basic.sh script by hand to add the path
to the system disk image they created. Rather than having them modify the
script, just grab it from the environment.
