Update net.py

Fix for https://github.com/mininet/mininet/issues/38 .

In mininet.net.Mininet.startTerms, we check that we create an xterm for a controller only for those controllers which are not remte i.e which are not instance of RemoteController.

This solution proposed has the slight overhead of running a filter on self.controllers but it saves the end user from unnecessary exceptions that calling it in mininet.term.makeTerm or mininet.Term.makeTerms could have caused.