pyqgis - How to show in a custom QGIS form the feature and/or geometry info?

I need to show point coordinates on the from dialog used to edit the attributes of a point.. I see that in the formOpen the dialog receives a layerId and a featureId, but no iface object, so how can get a provider to use the selectatId() and reach feature informations?

def formOpen(dialog,layerid,featureid):
...omissis...

(see here for a more detailed explanation of custom qgis forms)