oxenstored: reduce syslog call overhead

oxenstored: reduce syslog call overhead

We noticed that, if configured to use syslog as the logging backend, every
single line of access logging (via the syslog C binding) will call stat on
/etc/localtime for 3 times. The rational behind this is probably to detect any
timezone changes over time.

This is a considerable cost we'd like to avoid, given the intensiveness of our
access logging --- we log almost every xenstore status change (for good
reason). Also a running Xen host is rarely a mobile environment, so the little
benefit can hardly justify the cost.

Setting up the TZ environment varialbe can avoid stat calls.

Signed-off-by: Zheng Li <dev@zheng.li>
Reviewed-by: David Scott <dave.scott@citrix.com>