fix(frame_allocator): do not assume that the passed memory map never contains the overlapped descriptors

[toku-sa-n]

The description of UEFI's GetMemoryMap does not mention that each descriptor is exclusive. It means the frame allocator may return a frame that was allocated by the previous alloc call. FrameAllocator::init must merge descriptors that have common areas or reject a memory map that has a descriptor that is overlapped with other descriptors.