Ok I finally got it! Here it is:

First you must handle a bug in OpenERP 7. You can find what to do here: https://accounts.openerp.com/forum/Help-1/question/8598

The Bug has been fixed. Just update to last revision: http://bazaar.launchpad.net/~openerp/openerp-web/7.0/revision/4027

And then this module will work:

__init__.py:

import stock_location
import product

__openerp__.py:

{
    'name': 'nfx Stock Location',
    'version' : '0.1',
    'author' : 'Stefan Reisich, Rove.design GmbH',
    'website' : 'http://www.rove.de/',
    'description': 'Extends Stock Location functionality',
    'category': '',
    'depends': ['stock'],
    'data': ['stock_location.xml'],
    'installable': True,
}

product.py:

from openerp.osv import osv, fields


class product_product(osv.osv):
    _name = 'product.product'
    _inherit = 'product.product'

    def get_stock_locations(self, cr, uid, ids, field_names=None, arg=None, context=None):
        result = {}
        if not ids: return result

        context['only_with_stock'] = True

        for id in ids:
            context['product_id'] = id
            location_obj = self.pool.get('stock.location')
            result[id] = location_obj.search(cr, uid, [('usage', '=', 'internal')], context=context)

        return result


    _columns = {
        'stock_locations': fields.function(get_stock_locations, type='one2many', relation='stock.location', string='Stock by Location'),
    }

product_product()

stock_location.py:

from openerp.osv import osv


class stock_location(osv.osv):
    _name = "stock.location"
    _inherit = "stock.location"

    def search(self, cr, uid, args, offset=0, limit=None, order=None, context=None, count=False):
        if context is None:
            context = {}

        res_ids = super(stock_location, self).search(cr, uid, args, offset, limit, order, context=context, count=count)
        if context.get('only_with_stock', False) is True:
            loc_obj = self.browse(cr, uid, res_ids, context=context)
            res_ids = [x.id for x in loc_obj if x.stock_real>0]

        return res_ids


stock_location()

stock_location.xml:

<openerp>
    <data>

        <act_window
            context="{'product_id': active_id, 'only_with_stock': True}"
            id="act_stock_product_location_open"
            name="Stock by Location"
            res_model="stock.location"
            src_model="product.product"/>


        <record id="nfx_view_normal_procurement_locations_form" model="ir.ui.view">
            <field name="name">nfx_product.normal.procurement.locations.inherit</field>
            <field name="model">product.product</field>
            <field name="inherit_id" ref="stock.view_normal_procurement_locations_form"/>
            <field name="arch" type="xml">

                <group name="status" position="before">
                    <group name="nfx_locations" string="Locations" attrs="{'invisible': [('type', '=', 'service')]}" groups="base.group_user" colspan="2">

                        <field name="stock_locations" nolabel="1" context="{'product_id': active_id}">
                            <tree string="Stock Location">
                                <field name="complete_name"/>
                                <field name="stock_real"/>
                                <field name="stock_virtual"/>
                            </tree>
                        </field>

                    </group>
                </group>

            </field>
        </record>

    </data>
</openerp>