Rewritten XendLogging to not be a singleton class, and not have methods used

Rewritten XendLogging to not be a singleton class, and not have methods used
from nowhere at all.

Log to a temporary file if permission is denied to log to /var/log/xend.log,
as happens when you are not root.  Fixes bug #305.

Remove all the log-related garbage from XendRoot -- just about every module
was going straight to XendLogging.log already, so there was no need to have
XendRoot replicate this functionality.

Remove XendRoot.event_handler, which hasn't been in use since the EventServer
was removed.

Signed-off-by: Ewan Mellor <ewan@xensource.com>