This device will assist userspace to manage hotplug events.
A userspace application reads a DeviceEvent entry until the return value
is zero which indicates no events that are queued and waiting for
processing.
Trying to read with a buffer smaller than sizeof(DeviceEvent) results in
EOVERFLOW.
For now, there's no ioctl mechanism for this device but in the future an
acknowledgement mechanism can be implemented via ioctl(2) interface.
This singleton simplifies many aspects that we struggled with before:
1. There's no need to make derived classes of Device expose the
constructor as public anymore. The singleton is a friend of them, so he
can call the constructor. This solves the issue with try_create_device
helper neatly, hopefully for good.
2. Getting a reference of the NullDevice is now being done from this
singleton, which means that NullDevice no longer needs to use its own
singleton, and we can apply the try_create_device helper on it too :)
3. We can now defer registration completely after the Device constructor
which means the Device constructor is merely assigning the major and
minor numbers of the Device, and the try_create_device helper ensures it
calls the after_inserting method immediately after construction. This
creates a great opportunity to make registration more OOM-safe.
