change the format of the ami image type to vhdx

Prior this commit the ami image type produced raw.xz images. This was bad for
two reasons:

- The upload was broken because AWS doesn't support tar.xz format
- XZ compression is terribly slow

This commit changes the format to vhdx, which is supported by AWS and also
quite quick. See https://github.com/osbuild/osbuild-composer/issues/257
why vhdx was chosen.

Fixes #257

tools/test-case-generators/format-request-map.json

@@ -8,7 +8,7 @@
       "arch": "",
       "image-type": "ami",
       "repositories": [],
-      "filename": "image.raw.xz",
+      "filename": "image.vhdx",
       "blueprint": {}
     }
   },

tools/test-case-generators/generate-test-case

@@ -56,7 +56,7 @@ def main(test_case, store):
         image_file = os.path.join(store, "refs", output_id, test_case["compose-request"]["filename"])
 
         # we don't yet support image-info on directory trees
-        if boot_type in {"qemu-extract", "aws"}:
+        if boot_type in {"qemu-extract"}:
             fn, ex = os.path.splitext(image_file)
             if ex == ".xz":
                 with open(fn, "w") as f: