window.onload = function getVars() {
    // ...
    var x = locationInfo.lng;
    var y = locationInfo.lat;

    document.getElementById("formId:x").value = x;
    document.getElementById("formId:y").value = y;
}

<h:form id="formId">
    <h:inputHidden id="x" value="#{bean.x}" />
    <h:inputHidden id="y" value="#{bean.y}" />
</h:form>