block: Add AlignedBuffer

The block code repeatedly open-codes O_DIRECT alignment calculations and
bounce-buffer allocation at each I/O site. Add an AlignedBuffer struct
that handles the alignment and allocation in one place, using FileExt
(read_exact_at and write_all_at) for the I/O (so no need for custom libc
wrappers).

The caller creates an AlignedBuffer with an offset, length and
alignment, then uses read_from and write_to for aligned I/O and as_slice
and as_mut_slice to access the logical data portion within the aligned
region.

This is a lot like the AlignedBuf that was already existing in the QCOW2
code but is a more generalised version.

Assisted-by: Claude:Opus-4.6
Signed-off-by: Rob Bradford <rbradford@meta.com>

block/src/lib.rs

@@ -14,6 +14,7 @@ pub mod factory;
 #[path = "io/mod.rs"]
 mod io_impl;
 pub use io_impl::{async_io, fcntl, request};
+pub(crate) mod aligned_buffer;
 pub mod formats;
 mod sparse;
 use std::alloc::{Layout, alloc_zeroed};