Make sure the device event handler thread has started before returning from irecv_device_event_subscribe()
Make sure the device event handler thread has started before returning from irecv_device_event_subscribe()

This fixes a potential deadlock that might occur when irecv_device_event_unsubscribe()
is called before the device event handler thread has started, eventually
resulting in a deadlock.

libimobiledevice/libirecovery,libimobiledevice/libirecovery