platform: set default qemu image path
platform: set default qemu image path

When running kola inside the SDK the base image is easy to find. :)

marineam/mantle,quantum/coreos-mantle,mjg59/mantle,glevand/coreos--mantle,dm0-/mantle,mischief/mantle,dm0-/mantle,pbx0/mantle,mjg59/mantle,marineam/mantle,mischief/mantle,glevand/coreos--mantle,quantum/coreos-mantle