Catch duplicate instance names early
Currently, attempting to create an instance with a name that is already in use by a VM will result in a failure, but after the new base image is downloaded and extracted. We should check if the instance can be created before doing any of that.
As we don't have a central repository of known instances at this point, we should check:
- Does a VM exist by that name?
- Does a
qcow2
file exist by that name?