virtio: add missing mb() on enable notification

This fixes an issue dual to the one fixed by
patch 'virtio: add missing mb() on notification'
and applies on top.

In this case, to enable vq kick to exit to host,
qemu writes out used flag then reads the
avail index. if these are reordered we get a race:

    host avail index read: ring is empty
    		guest avail index write
    		guest flag read: exit disabled
    host used flag write: enable exit

which results in a lost exit: host will never be notified about the
avail index update.  Again, happens in the field but only seems to
trigger on some specific hardware.

Insert an smp_mb barrier operation to ensure the correct ordering.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
