block: vhdx: Use AlignedFile for O_DIRECT-safe I/O

By redirecting VHDx I/O through the AlignedFile the required RMW
semantics can be achieved for writes less than the logical block size
whilt reusing the same logic used for other backend implementations.

Assisted-by: Claude:Opus-4.8
Signed-off-by: Rob Bradford <rbradford@meta.com>

fuzz/fuzz_targets/vhdx.rs

@@ -22,7 +22,7 @@ fuzz_target!(|bytes: &[u8]| -> Corpus {
     disk_file.write_all(&bytes[..]).unwrap();
     disk_file.seek(SeekFrom::Start(0)).unwrap();
 
-    let mut vhdx = match Vhdx::new(disk_file) {
+    let mut vhdx = match Vhdx::new(disk_file, false) {
         Ok(vhdx) => vhdx,
         Err(_) => return Corpus::Reject,
     };