Make LoadKernel() pass back the kernel partition's UniqueGuid.

LoadKernel already returns the partition number for the selected kernel.
This change makes it also return the GPT Entry's UniqueGuid, which will
eventually be passed to the kernel itself, so the kernel can determine which
of several possible devices it has booted from. It doesn't know for certain
because the BIOS and the kernel may enumerate the devices in a different


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