Before this patch only log messages with precisely priority

Before this patch only log messages with precisely priority
LOG_WARNING (or precisely LOG_DEBUG if -v) would be logged.

xenconsoled only actually logs using LOG_ERR and LOG_DEBUG so for the
most part we would have been logging absolutely nothing up until now.

Linux provides a LOG_UPTO macro but I'm not sure how portable that is
so I have opencoded the list of levels.

Do I get some sort of prize for fixing a bug introduced in 2005?

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>