libvhost-user: Fix wrong type of argument to formatting function (reported by LGTM)

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-Id: <20220422070144.1043697-2-sw@weilnetz.de>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221126152507.283271-2-sw@weilnetz.de>