Don't bother messing with minphys, we always need to support
transfers of size MAXBSIZE. If the number of DMA segments
supported by the device is too low, don't use it.

Discussion at http://marc.info/?l=openbsd-tech&m=140978303923966&w=2

imported to Bitrig by patrick@, ok'd by pedro@
Add missing memory barriers to virtio.

imported to Bitrig and adapted to C11 atomics by patrick@, ok'd by pedro@
publish_avail_idx does not read the used ring, no need to call
bus_dmamap_sync for it.

imported to Bitrig by patrick@, ok'd by pedro@