Add trace functions in vhost-vdpa.c.

All traces from this file can be enabled with '-trace vhost_vdpa*'.

Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20200925091055.186023-3-lvivier@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>