vmwareapi: raise on get_console_output

Commit 2ce3461595a490c9a02272ee4a1ab1eb55c27f60 updated
get_console_output() in the vmwareapi driver to return a static string.
Since this isn't supported, I think the most correct thing to do here is
to raise NotImplementedError.  That will get properly propogated back
through the API.

The original change was concerned with getting a friendly message to the
user.  That should be done by the consumer of the API.  For example, if
Horizon got that as an error back, it could present a special message to
the user indicating that the operation is not supported for that
instance.

Change-Id: Ia17330b9084bf3e82219a0c143485174c3e8f6db
Merge "vmwareapi: raise on get_console_output"