msix: unset PCIDevice::msix_vector_poll_notifier in rollback

msix: unset PCIDevice::msix_vector_poll_notifier in rollback

In the rollback in msix_set_vector_notifiers(), original patch forgot to
undo msix_vector_poll_notifier pointer.

Fixes: bbef882cc193 ("msi: add API to get notified about pending bit poll")
Signed-off-by: Robert Hoo <robert.hoo.linux@gmail.com>
Message-Id: <20231113081349.1307-1-robert.hoo.linux@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>