Feature/safe icicle forced external

I'm not really convinced this should be merged as-is, but it demonstrates at least one way to use existing libvirt functionality to allow ICICLE generation without modifying the underlying image.

This is of interest when using override kickstart files for customization, rather than the content of the TDL.  In the appliance-creation replacement use case it's important not to actually boot the guest after Anaconda has finished creating it.

Discuss.