[FIX] addons: fix usage of decorators `api.v7`/`api.v8`

Specifically, when one API implementation calls the other one, it has to call
the method *from the same class*.  Otherwise, overriding the method may result
in an infinite recursion.  Consider:

    class A(Model):
        _name = 'stuff'

        @api.v8
        def foo(self):
            return 42

        @api.v7
        def foo(self, cr, uid, context=None):
            return self.browse(cr, uid, [], context).foo()

    class B(Model):
        _inherit = 'stuff'

        def foo(self, cr, uid, context=None):
            return super(B, self).foo(cr, uid, context=context) + 1

and now call: `env['stuff'].foo()`.  This invokes `B.foo` (new-API), which
calls `B.foo` (old-API), which calls `A.foo` (old-API), which calls `B.foo`
(new-API) instead of `A.foo`!

This issue would not be present if old-API `A.foo` was defined as:

        @api.v7
        def foo(self, cr, uid, context=None):
            return A.foo(self.browse(cr, uid, [], context))