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>

block/src/factory.rs

@@ -114,7 +114,7 @@ fn open_vhdx(
 ) -> BlockResult<Box<dyn AsyncFullDiskFile>> {
     info!("Opening VHDX disk file with synchronous backend");
     Ok(Box::new(
-        VhdxDisk::new(file).map_err(|e| e.with_path(options.path))?,
+        VhdxDisk::new(file, options.direct).map_err(|e| e.with_path(options.path))?,
     ))
 }