Stick the pid file into /run/sendsigs.omit.d. If xenstored is gone, any
attempt to use "xl" will hang un-killably, at least on Xen 4.4.

Note that this does not help shutdown to single user, which runs a different
"killprocs" script as it enters single-user. That kills all processes with
no omissions (/etc/rc1.d/S30killprocs)! We may want to consider running the
Xen start script following that to at least re-start xenstored (and which
will free up any hung processes).