Don't set the firmware path in 3 different places
Updates for systemd 198/199
Set the kernel.poweroff_cmd sysctl

This allows Xen (and EC2) to power off an instance properly.  We had
this before (see aeb89fc753c6f95c9d143e6f5346f92e4a02fa67), but it got
lost in the systemd migration.
Clear /proc/sys/kernel/hotplug again

Otherwise the kernel will pointlessly try to invoke /sbin/hotplug all
the time.  Another feature lost in the systemd switch.