Ensure domain is taken down when we exhaust retries

This was just an oversight - we don't destroy and undefine the
'createhdds' domain on the path where the final retry to create
an image times out, we just wipe the temp file and exit. So we
leave a qemu process sitting around until createhdds runs again
(and, on openQA production, exhausting swap space).

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

createhdds.py

@@ -305,6 +305,9 @@ class VirtInstallImage(object):
                 ret = subprocess.call(args, timeout=3600)
             except subprocess.TimeoutExpired:
                 logger.warning("Image creation timed out!")
+                dom.destroy()
+                dom.undefine()
+                conn.close()
                 if os.path.isfile(tmpfile):
                     os.remove(tmpfile)
                 if retries: