I'm not sure to have well understood the question.
But if you want a generic iframe preview,
a tricky way can be to use a template and store the computed template in a field.

Here an example of how it could be made (to test)...


from openerp import api, fields, models

Model :

class ItemStocks(models.Model):
    _name = 'item.stocks'
     item_url = fields.Char('View Item')
     ItemStocks()

     iframe = fields.Html('Embedded Webpage', compute='_compute_iframe', sanitize=False, strip_style=False )
  
    @api.multi
     def _compute_iframe(self):
        for item_stock in self:
            url = item_stock.item_url
            template = self.env.ref('<your_module_name>.framed_page')
            item_stock.framed_page_rendered = template.render({ 'url' : url })  

View:

<template id='framed_page'>
     <iframe t-att-src="url"
     marginheight="0" marginwidth="0" frameborder="0"
     width="100%" height="1000"></iframe>
</template>


<record id="view_item_stocks_form" model="ir.ui.view">
     <field name="name">item.stocks.form</field>
     <field name="model">item.stocks</field>
     <field name="arch" type="xml">
         <form string="Item Stocks"> ...
            <page string="Live Site">
                 <form string="Embedded Webpage" version="7.0" edit="false">
                <field name="framed_page_rendered" widget="html"/>
                 </form>
         </page> ...
        </form>
     </field>
</record>