vhost-vdpa: Fix the wrong input in config_cb

commit bcef9356 upstream.

Fix the wrong input in for config_cb. In function vhost_vdpa_config_cb,
the input cb.private was used as struct vhost_vdpa, so the input was
wrong here, fix this issue

Fixes: 776f3950

 ("vhost_vdpa: Support config interrupt in vdpa")
Signed-off-by: Cindy Lu <lulu@redhat.com>
Link: https://lore.kernel.org/r/20210929090933.20465-1-lulu@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>