virtio_ring: update weak barriers to use virt_xxx

virtio ring uses smp_wmb on SMP and wmb on !SMP,
the reason for the later being that it might be
talking to another kernel on the same SMP machine.

This is exactly what virt_xxx barriers do,
so switch to these instead of homegrown ifdef hacks.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Alexander Duyck <alexander.duyck@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>