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": {}
     }
   },