@scpcom
Also hit that error there  is an modification required in /opt/xensource/installer/answerfile.py
def processAnswerfile(self):
        xelogging.log("Processing XML answerfile for %s." % self.operation)
        if self.operation == 'installation':
            install_type = getStrAttribute(self.top_node, ['mode'], default = 'fresh')
            results['netinstall-gpg-check'] = getBoolAttribute(self.top_node, ['netinstall-gpg-check'], default = True)
            if install_type == "fresh":
                results = self.parseFreshInstall()
            elif install_type == "reinstall":
                results = self.parseReinstall()
            elif install_type == "upgrade":
                results = self.parseUpgrade()
            else:
                raise AnswerfileException, "Unknown mode, %s" % install_type
            results.update(self.parseCommon())
        elif self.operation == 'restore':
            results = self.parseRestore()
        
        return results
Should be
def processAnswerfile(self):
        xelogging.log("Processing XML answerfile for %s." % self.operation)
        if self.operation == 'installation':
            install_type = getStrAttribute(self.top_node, ['mode'], default = 'fresh')
            results = {}
            results['netinstall-gpg-check'] = getBoolAttribute(self.top_node, ['netinstall-gpg-check'], default = True)
            if install_type == "fresh":
                results.update(self.parseFreshInstall())
            elif install_type == "reinstall":
                results.update(self.parseReinstall())
            elif install_type == "upgrade":
                results.update(self.parseUpgrade())
            else:
                raise AnswerfileException, "Unknown mode, %s" % install_type
            results.update(self.parseCommon())
        elif self.operation == 'restore':
            results = self.parseRestore()
        
        return results
In my case i started from the iso and passed an answerfile. Had to boot into manual mode switch to console #2 (ALT+F2) made the modification with vi, killed the process /opt/xensource/installer/init and launched it again using the answerfile and install options
/opt/xensource/installer/init --answerfile=[path to your answer file] --install
In your case you may have to modify the file inside the install.img at your url location.
Hope that modification will make it into the final iso.