virtio: clean up memory barrier usage

virtio: clean up memory barrier usage

cleanup memory barrier usage bringing it
in sync with what linux guest does.
The rules are simple:
- read barrier after index read
- write barrier before index write
Also, call macros smp_rmb/smp_wmb to stress
we are not syncing with a real io device here.

While I don't think compiler is crazy/powerful
enough to reorder these, anyway, the bogus
barriers we currently have in code will confuse
anyone who tries to copy/reuse it.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>