This commit tweaks the handling of the `--output-name` option so
that is a name with the same extension as the image is passed that
is just silently ignored. Its a common issue that first time
users run:
```console
$ image-builder build --output-name foo.qcow2 qcow2
```
which currently leads to a foo.qcow2.qcow2. With this commit
the expected "foo.qcow2" will appear.
d00e76ced1