text: -- remove deleted quota from images and also Guid.Empty
  UPDATE images
  SET    quota_id = NULL
  WHERE  quota_id NOT IN (SELECT id FROM quota);
  -- remove deleted quota from vm_static
  UPDATE vm_static
  SET    quota_id = NULL
  WHERE  quota_id NOT IN (SELECT id FROM quota);
  -- when removing quota, set quota_id to null in images and vm_static
  ALTER TABLE images ADD CONSTRAINT fk_images_quota FOREIGN KEY (quota_id) REFERENCES quota(id) ON DELETE SET NULL;
  ALTER TABLE vm_static ADD CONSTRAINT fk_vm_static_quota FOREIGN KEY (quota_id) REFERENCES quota(id) ON DELETE SET NULL;
  