Some plugins have devices with more than one protocol. Logically the protocol
belongs to the device, not the plugin, and in the future we could use this to
further check firmware that's about to be deployed.
This is also not exported into libfwupd (yet?) as it's remains a debug-feature
only -- protocols are not actually required for devices to be added.
This will allow us to switch to the new firmware version during shutdown when
the filesystems have been remounted readonly.
Activating manually means we don't have to get the user to shutdown and then
do a fresh power-on, rather than the more usual restart.