venus should not use vkWaitForFences() for notification that a fence is signaled
With mesa/mesa!21973 and corresponding kernel side support, this will cause frequency boost of the GPU at a time when we do not know if the guest is actually waiting for the fence. Instead a dma-fence fd should be exported and use poll()
to wait.
virgl and native-context should already be using poll(dma_fence_fd)
so they should be ok.