util: qemu: Allow for different approaches to format JSON arrays

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.