<?xml version="1.0"?>
<data><xpath expr="//field[@name='category_id']" position="before">
                <field name="website_id" invisible="1"/>
            </xpath>

            <xpath expr="//field[@name='category_id']" position="attributes">
                <attribute name="domain">['|', ('website_id', '=', website_id), ('website_id', '=', False)]</attribute>
            </xpath>

        </data>