oxenstored: enable logging via syslog, if specified in the config file.

oxenstored: enable logging via syslog, if specified in the config file.

To log to files the config file should contain:
   xenstored-log-file = /var/log/xenstored.log
   access-log-file = /var/log/xenstored-access.log

(These two files are still the built-in defaults. The log format is
unchanged.)

To log to syslog the config file should contain:
   xenstored-log-file = syslog:<facility>
   access-log-file = syslog:<facility>

where <facility> is the syslog facility to use (e.g. 'daemon' 'local2')

Signed-off-by: David Scott <dave.scott@eu.citrix.com>