util: qemu: Allow for different approaches to format JSON arrays
    
    For use with memory hotplug virQEMUBuildCommandLineJSONRecurse attempted
    to format JSON arrays as bitmap on the command line. Make the formatter
    function configurable so that it can be reused with different syntaxes
    of arrays such as numbered arrays for use with disk sources.
    
    This patch extracts the code and adds a parameter for the function that
    will allow to plug in different formatters.