efi-api: fix unaligned access in efi_guid_to_id128()

efi-api: fix unaligned access in efi_guid_to_id128()

EFI_GUID requires 4-byte alignment due to its uint32_t Data1 field, but
callers may pass pointers at arbitrary offsets into serialized EFI
variable buffers (e.g. bootctl walking BootXXXX entries). UBSan flagged
the misaligned member access; the old comment claiming the struct was
packed was wrong. Copy the bytes into an aligned local first.

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>