Do not emit spurious ::device-removed events when emulating

[hughsie]

If we load new emulation data we have to do horrible tricks in the caller to avoid handling the remove event when loading new event data.

Make the caller (fwupd) much simpler by emitting the correct signals.

Also add a ::device-changed signal that we can use to reload the device events.

[hughsie]

@fdanis-oss this means we can completely remove the removed state machine in fwupd which is a constant source of confusion and complexity.