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

internal/upload/awsupload/awsupload.go

@@ -109,6 +109,7 @@ func (a *AWS) Register(name, bucket, key string) (*string, error) {
 					S3Bucket: aws.String(bucket),
 					S3Key:    aws.String(key),
 				},
+				Format: aws.String("vhdx"),
 			},
 		},
 	)