virtio: Use global memory barrier macros

virtio: Use global memory barrier macros

The virtio code uses wmb() macros in several places, as required by the
SMP-aware virtio protocol.  However the wmb() macro is locally defined
to be a compiler barrier only.  This is probably sufficient on x86
due to its strong storage ordering model, but it certainly isn't on other
platforms, such as ppc.

In any case, qemu already has some globally defined memory barrier macros
in qemu-barrier.h.  This patch, therefore converts virtio.c to use those
barrier macros.  The macros in qemu-barrier.h are also wrong (or at least,
safe for x86 only) but this way at least there's only one place to fix
them.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>