Pre-filter the pending LuDeviceBootloaderRequest's after they are parsed so
that we know exactly how many packets need to be sent to the hardware.
As shown in https://github.com/hughsie/fwupd/issues/155, people get worried
if the progress bar finishes before 100% as they'll wonder if all the writing
completed successfully.
Although the device does send a response, in some cases the Linux USB stack
processes the disconnect before the reply is processed.
Resolves: https://github.com/hughsie/fwupd/issues/122
