builder: use cloud api

Composer now[1] has integrated the koji API into the "cloud API"
and thus we can use this more general purpose and powerful API
instead of using the specialized koji API endpoint.
Adapt the request and response structures as well as the unit
tests to use that.

[1] PR #2214, commit 11e2ae45284bfb0d89ef1c1e0d2aa4ae230ea573

plugins/cli/osbuild.py

@@ -27,7 +27,7 @@ def parse_args(argv):
                             "RPMs in the image. May be used multiple times. The "
                             "build tag repo associated with the target is the default."))
     parser.add_option("--image-type", metavar="TYPE",
-                      help='Request an image-type [default: qcow2]',
+                      help='Request an image-type [default: guest-image]',
                       type=str, action="append", default=[])
     parser.add_option("--skip-tag", action="store_true",
                       help="Do not attempt to tag package")
@@ -69,7 +69,7 @@ def handle_osbuild_image(options, session, argv):
     distro, image_types = args.distro, args.image_type
 
     if not image_types:
-        image_types = ["qcow2"]
+        image_types = ["guest-image"]
 
     opts = {}