Add PowerPC support in createhdds script and json
    
    Assumption createhdds executed on a PowerPC ppc64le host
    to create the PowerPC specific images.
    
    Detect current CPU arch of host machine to create virt-install images
    only for supported architectures. (hardcoded lists)
    
    hdds.json specific changes for PowerPC
    * no desktop or kde images
    
    Signed-off-by: Michel Normand <normand@linux.vnet.ibm.com>