qemu: command: Properly format disk 'debug' attribute
    
    Move the setup of the disk attribute to the disk source prepare function
    which will allow proper usage with JSON props and move the fallback
    (legacy) generating code into the block which is executed with legacy
    options.
    
    As a side-effect of this change we can clean up propagation of 'cfg'
    into the command generator.
    
    Also it's nice to see that the test output is the same even when the
    value is generated in a different place.