XenAPI: Fix race condition on tailing stack.log

Stack.log is created by stack.sh, which is started by run.sh.  Using
the pid from run.sh doesn't mean that stack.log has been created yet
so wait for the log to be created before tailing

Change-Id: I86e563478949cb7bbe0d1afd80b790ba70328e3a
Merge "XenAPI: Fix race condition on tailing stack.log"