allow known ssh key for QEMU image
Using the QEMU file for most programatic use cases is difficult without a ssh key pre-set.
Either using the image in a vagrant qemu backend, or directly via qemu-system-x86_64 -display none -daemonize
makes it very difficult to interact with the image since there's no way to ssh to the instance or attach qemu to std i/o without too much unrelated code (as this repo does, btw)
This change just add (and centralizes in common) the code to create the user with a known public ssh key.