hw/virtio: fix vhost_user_read tracepoint

hw/virtio: fix vhost_user_read tracepoint

As reads happen in the callback we were never seeing them. We only
really care about the header so move the tracepoint to when the header
is complete.

Fixes: 6ca6d8ee9d (hw/virtio: add vhost_user_[read|write] trace points)
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220728135503.1060062-5-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>