An eventfd is used to signal the data loop from the libcamera request
completion event handler. Previously, this eventfd was created and
installed after the camera has been started and requests were queued.
This is problematic because it creates a small time frame where the
libcamera request completion handler will run in a state where the
eventfd is not fully set up.
Fix that by settup up the eventfd before the camera is started.
e0e8bf083d