libvhost-user: Add format attribute to local function vu_panic

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220422070144.1043697-4-sw@weilnetz.de>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221126152507.283271-5-sw@weilnetz.de>