Ability to add custom cli flags

[michaelschapira]

I want to experiment with CoreOS. To boot / install and ignition file needs to be provided.
As per documentation several CLI parameters need to be provided :
--qemu-commandline="-fw_cfg name=opt/com.coreos/config,file=${IGNITION_CONFIG}"

Thanks.

[garrett]

I've created an issue based on this @ #1835 as I think we need it to properly support creating CoreOS VMs.