vhost-user-test: fix crash with glib < 2.36

The prepare callback needs to be implemented with glib < 2.36,
quoting glib documentation:
"Since 2.36 this may be NULL, in which case the effect is as if the
function always returns FALSE with a timeout of -1."

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
