Allow disabling ephemeral disk formatting

[This came out of a downstream discussion with Red Hat OSP developers.]

Currently, libguestfs is only used in two places:

- file injection and
- ephemeral disk creation

The initial suggestion here is to add an option such as `default_ephemeral_format=unformatted`. This allows optionally disable libguestfs usage in ephemeral disk creation, if you don't need it. File injection is already optional.

An example use-case of this is: if you're shipping Nova Compute services in a container, you might want to reducing dependencies that are pulled in by libguestfs, if you have no need for it in the container. This reduces the Nova Compute container footprint.