[FIX] product: Put product_variant_count on template form view.

Was missing to be used in domain added at 92d8f15a.
Fixes #2570

addons/product/product_view.xml

@@ -50,6 +50,7 @@
             <field name="arch" type="xml">
                 <form string="Product">
                     <field name="is_product_variant" invisible="1"/>
+                    <field name='product_variant_count' invisible='1'/>
                     <sheet>
                         <field name="id" invisible="True"/>
                         <div class="oe_right oe_button_box" style="width: 300px;" name="buttons">