Currently we have three virt-sandbox-image commands

 - 'create' - downloads a template and creates qcow2
 - 'delete' - deletes template qcow2 files
 - 'run' - runs an instance of a template

The 'run' command is generating a transient guest which
disappears when it stops. We want to have the ability to
create / delete persistent guests later, for which the
command names "create" and "delete" are a natural fit.

So to avoid clash, rename the existing "create" command
to "prepare" and the "delete" command to "purge"