virtio-gpu: fix information leak in getting capset info dispatch

In virgl_cmd_get_capset_info dispatch function, the 'resp' hasn't
been full initialized before writing to the guest. This will leak
the 'resp.padding' and 'resp.hdr.padding' fieds to the guest. This
patch fix this issue.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Message-id: 5818661e.0860240a.77264.7a56@mx.google.com
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
