Add missing memory barriers to virtio.
Add missing memory barriers to virtio.

The virtio_membar_sync() calls are required even on x86. The *_producer()
and *_consumer() calls are only relevant on architectures with less strict
memory ordering.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars