This adds a function to get the emulated or real Bluetooth device for
an active emulation instance. This lets us deduplicate all the
`ios->GetDeviceByName("/dev/usb/oh1/57e/305")` calls that are currently
scattered in the codebase and ensures Bluetooth passthrough is being
handled correctly.
This also fixes the broken check in WiimoteCommon::UpdateSource.
There was a confusion between "emulated Bluetooth" (as opposed to
"real Bluetooth" aka Bluetooth passthrough) and "emulated Wiimote".