This adds RemoveDevice() to ControllerInterface, fixes ExpressionParser
and some other code to support device removals without crashing,
and adds an IsValid() method to Device, to prepare for hotplugging.
Small cleanup by using std::shared_ptr and getting rid of
ciface.Devices() which just returned the m_devices (which defeats the
point of making m_devices protected).
Incidentally, this should make the code safer when we have
different threads accessing devices in the future (for hotplug?).
A lot of code use Device references directly so there is
no easy way to remove FindDevice() and make those unique_ptrs.
