tools/hotplug: allow tuning of xenwatchdogd arguments

tools/hotplug: allow tuning of xenwatchdogd arguments

Currently the arguments for xenwatchdogd are hardcoded with 15s
keep-alive interval and 30s timeout.

It is not possible to tweak these values via
/etc/systemd/system/xen-watchdog.service.d/*.conf because ExecStart
can not be replaced. The only option would be a private copy
/etc/systemd/system/xen-watchdog.service, which may get out of sync
with the Xen provided xen-watchdog.service.

Adjust the service file to recognize XENWATCHDOGD_ARGS= in a
private unit configuration file.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Wei Liu <wl@xen.org>