boot: use EFI_DISK_IO_PROTOCOL instead of EFI_BLOCK_IO_PROTOCOL for disk reads

boot: use EFI_DISK_IO_PROTOCOL instead of EFI_BLOCK_IO_PROTOCOL for disk reads

EFI_DISK_IO_PROTOCOL (UEFI spec section 13.7,
https://uefi.org/specs/UEFI/2.10/13_Protocols_Media_Access.html#disk-i-o-protocol)
supports reads at arbitrary byte offsets with no alignment requirements on the
buffer. The UEFI spec mandates that firmware produces this protocol on every
handle that also has EFI_BLOCK_IO_PROTOCOL, so it is always available.

This is a better fit than EFI_BLOCK_IO_PROTOCOL for our GPT parsing and
BitLocker detection because Block I/O requires that both the read offset (LBA)
and the buffer are aligned to the media's IoAlign value. Meeting that
constraint forces us to use xmalloc_aligned_pages() with
PHYSICAL_ADDRESS_TO_POINTER(), page-granularity allocations, and manual size
rounding (ALIGN_TO). Disk I/O handles all of that internally, so callers can
use plain xmalloc() or even stack buffers and read exactly the number of bytes
they need.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>