XenServer: the cron job shouldn't print debug text into stderr

one cron job attempts to print debug text into stderr; so the file
of /root/dead.letter gets created and its size grows continuously.
It could eventually threaten dom0 disk space. Maybe there are two
solutions: one is to redirect the output to a specific log file;
and rotate log files in the script. And the other one is simply
to redirect the output /dev/null. By considering the function of
this cron job and the printed contents are straight and simple,
this patch set goes with the later solution.

Change-Id: I4875e5e3837e6f0249e314c6c5f408c79145c6c1
Closes-Bug: 1503966

lib/nova_plugins/hypervisor-xenserver

@@ -79,7 +79,7 @@ function configure_nova_hypervisor {
 
     # Create a cron job that will rotate guest logs
     $ssh_dom0 crontab - << CRONTAB
-* * * * * /root/rotate_xen_guest_logs.sh
+* * * * * /root/rotate_xen_guest_logs.sh >/dev/null 2>&1
 CRONTAB
 
     # Create directories for kernels and images