cmdlib._CreateDisks fix a broken result.Raise

The format string has the ": %s" at the end, but no argument is passed,
which of course raises a TypeError. Removing ": %s" as it's added by
the RpcResult Raise() method anyway.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>