vdi: Avoid bitrot of debugging code

vdi: Avoid bitrot of debugging code

Rework the debug define so that we always get -Wformat checking,
even when debugging is disabled.

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Weil <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>