OS X and OS X Server will, at some point complain that they are not running on "real" Apple hardware. This is because a Fusion virtual machine is not totally representative of the underlying hardware. If you want to keep trying until you reach a blocking point, you might try setting the Guest OS to Darwin, which is what OS X is based on.
For anyone that might chime in, yes there are unauthorized, "cracked", copies of OS X available on pirate networks that may run under Fusion.