Returned from the OpenProtocol operation, the grub_efi_block_io_media
structure contains the io_align field, specifying the minimum alignment
required for buffers used in any data transfers with the device.
Make grub_efidisk_readwrite() allocate a temporary buffer, aligned to
this boundary, if the buffer passed to it does not already meet the
requirements.
Also sanity check the io_align field in grub_efidisk_open() for
power-of-two-ness and bail if invalid.
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