tools/hotplug: systemd: Don't ever kill xenstored

tools/hotplug: systemd: Don't ever kill xenstored

Don't kill xenstored as part of the usual service shutdown process to
prevent hangs on shutdown where the kernel tries to unplug a VIF
after xenstored has exited.

In an ideal case with all guests cooperating, xendomains will have shut
down all guests before xenstored is killed.

However in the uncooperative case, malicious or crashed guests may still
be running after xendomains has exited and this should not block the
shutdown/reboot of dom0.

Xenstored has no state to sync to disk, and never used to be killed in
the sysvinit case; observe the warning in xencommons.  Our testing has
shown regressions caused by the change in behaviour between sysvinit and
systemd when it comes to killing xenstored.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
[ ijc -- added systemd to title ]