qemu: command: Add infrastructure for object specified disk sources
    
    To allow richer definitions of disk sources add infrastructure that will
    allow to register functionst generating a JSON object based definition.
    
    This infrastructure will then convert the definition to the proper
    command line syntax and use it in cases where it's necessary. This will
    allow to keep legacy definitions for back-compat when possible and use
    the new definitions for the configurations requiring them.