Executes `virtualenv` to create a new environment.

def _create(self):
        """Executes `virtualenv` to create a new environment."""
        if self.readonly:
            raise VirtualenvReadonlyException()
        args = ['virtualenv']
        if self.system_site_packages:
            args.append('--system-site-packages')
        if self.python is None:
            args.append(self.name)
        else:
            args.extend(['-p', self.python, self.name])
        proc = subprocess.Popen(args, cwd=self.root, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
        output, error = proc.communicate()
        returncode = proc.returncode
        if returncode:
            raise VirtualenvCreationException((returncode, output, self.name))
        self._write_to_log(output, truncate=True)
        self._write_to_error(error, truncate=True)