Don't produce ppc64 images for Fedora > 28

ppc64 (as opposed to ppc64le) was retired in Fedora 29. Tweaking
hdds.json to reflect this now is a bit awkward, so instead let's
leave it as-is, but add code to createhdds to not really include
a ppc64 image in the expected list if it's for F29 or later.
Once F28 goes EOL we can just drop all ppc64 from hdds.json and
lose this code.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

createhdds.py

@@ -492,6 +492,11 @@ def get_virtinstall_images(imggrp, nextrel=None, releases=None):
             rels = [release]
         for arch in arches:
             for rel in rels:
+                # ppc64 was retired for f29; let's filter out all ppc64
+                # images for f29 and later. Once f28 is EOL we can just
+                # ditch them from hdds.json and remove this
+                if arch == 'ppc64' and (rel == 'rawhide' or int(rel) > 28):
+                    continue
                 imgs.append(
                     VirtInstallImage(name, rel, arch, variant=variant, size=size, imgver=imgver,
                                      maxage=maxage))