This will make qemu aware of the device used buffers, allowing it to
write the guest memory with its contents if needed.
Since the use of vhost_virtqueue_start can unmasks and discard call
events, vhost_virtqueue_start should be modified in one of these ways:
* Split in two: One of them uses all logic to start a queue with no
side effects for the guest, and another one tha actually assumes that
the guest has just started the device. Vdpa should use just the
former.
* Actually store and check if the guest notifier is masked, and do it
conditionally.
* Left as it is, and duplicate all the logic in vhost-vdpa.