block: Move VHDX format files into formats/vhdx/

Move VHDX format implementation into a structured directory layout:

  vhdx/mod.rs        -> formats/vhdx/internal/mod.rs      (Vhdx)
  vhdx/vhdx_bat.rs   -> formats/vhdx/internal/bat.rs
  vhdx/vhdx_header.rs -> formats/vhdx/internal/header.rs
  vhdx/vhdx_io.rs    -> formats/vhdx/internal/io.rs
  vhdx/vhdx_metadata.rs -> formats/vhdx/internal/metadata.rs
  vhdx_sync.rs       -> formats/vhdx/mod.rs               (VhdxDisk)

Extract VhdxSync from vhdx_sync.rs into formats/vhdx/worker/sync.rs.
Drop the vhdx_ prefix from internal file names since the parent
directory already provides the namespace. Update all internal cross
references to use the new module paths. Re-export formats::vhdx as
vhdx_sync in lib.rs for backward compatibility.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/formats/mod.rs

@@ -9,3 +9,4 @@
 
 pub mod raw;
 pub mod vhd;
+pub mod vhdx;