Plugins are allowed to 'opt-out' of this behaviour using _RULE_INHIBITS_IDLE.
This should be used where waking up the hardware to coldplug is expensive,
either from a power consumption point of view, or if other artifacts are going
to be seem -- for instance if the screen flickers when probing display devices.
This functionality is also inhibited when the actual upgrade is happening,
for obvious reasons.
Admins can turn off this auto-sleep behaviour by editing the daemon.conf file.
Fixes https://github.com/hughsie/fwupd/issues/417
