Legacy's `Machine#id=()` has an important side effect when a nil ID is
specified - it clears the contents of the machine's DataDir.
We mirrored this behavior over to gogo, with a subtle difference - we
deleted the whole DataDir vs just its children.
It turns out the Docker provider relies on the DataDir being
cleared-but-not-removed by doing a SetId dance in its InitState action.
(see 1e6259dd00d702f83048c75c5c229ce6494c4c6e).
So here we need to mirror that behavior in order for the Docker provider
to work properly.
The type needs a string pointer now. We may end up wanting a string
pointer making convenience method in the SDK at some point, but for now
I punted on that question and just made one for the test.
The uuid is the public identifier vs the resource id which should
be for internal operations.
The target uuid should correspond to the machine id (given by
the provider)
