vdpa_sim: use weak barriers

vDPA simulators are software emulated device, so let's switch to use
weak barriers to avoid extra overhead in the driver.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20221221062146.15356-1-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>