Si Ya habia googleado el problema, era puntual en mi controlador: se debia llamar al metodo createcreateform en vez de createform.
Código:
public function createAction()
{
$entity = new User();
$request = $this->getRequest();
//$form = $this->createForm(new UserType(), $entity);
// --------------
$form = $this->createCreateForm($entity);
$form->submit($request);
/*
*/
if ($form->isValid()) {
//establecemos la contraseña: --------------------------
$this->setSecurePassword($entity);
$em = $this->getDoctrine()->getEntityManager();
$em->persist($entity);
$em->flush();
return $this->redirect($this->generateUrl('admin_user_show', array('id' => $entity->getId())));
}
return array(
'entity' => $entity,
'form' => $form->createView()
);
}