Be more explicit about the names of Entropy devices
Revert b872b93d9056b3 to only expose a `default_entropy` for now.

We will eventually need a xen-specific entropy device to specify
the backend configuration (for `rndback`), but for now the module
just self-seeds as best it can.
Merge pull request #260 from avsm/entropy-1.2

Be more explicit about the names of Entropy devices