virtio-pmem: ignore empty queue notifications

virtio_pmem_flush() treats a NULL return from virtqueue_pop() as a fatal
error and calls virtio_error(), which puts the device into NEEDS_RESET.

However, virtqueue handlers can be invoked when no element is available,
so an empty queue should be handled as a benign no-op.

With a Linux guest this avoids spurious NEEDS_RESET and the resulting
-EIO propagation (e.g. EXT4 journal abort and remount-ro).

Signed-off-by: Li Chen <me@linux.beauty>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260106083859.380338-1-me@linux.beauty>
(cherry picked from commit efd581a8cd4405ca183ecd017072b0c878802d69)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/virtio/virtio-pmem.c

@@ -74,7 +74,6 @@ static void virtio_pmem_flush(VirtIODevice *vdev, VirtQueue *vq)
     trace_virtio_pmem_flush_request();
     req_data = virtqueue_pop(vq, sizeof(VirtIODeviceRequest));
     if (!req_data) {
-        virtio_error(vdev, "virtio-pmem missing request data");
         return;
     }