block: Move VHD format files into formats/vhd/

Move VHD format implementation into a structured directory layout:

  fixed_vhd.rs       -> formats/vhd/internal/fixed.rs     (FixedVhd)
  fixed_vhd_disk.rs  -> formats/vhd/mod.rs                (VhdDisk)
  vhd.rs             -> formats/vhd/internal/footer.rs     (VhdFooter)
  fixed_vhd_sync.rs  -> formats/vhd/worker/sync.rs         (FixedVhdSync)
  fixed_vhd_async.rs -> formats/vhd/worker/async_uring.rs  (FixedVhdAsync)

Add #[allow(dead_code)] to VhdFooter struct and impl because the
module is now pub(crate) and the compiler can see that several
fields and getters are only exercised by unit tests. Re-export
formats::vhd as fixed_vhd_disk in lib.rs for backward
compatibility with external consumers.

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

block/src/formats/mod.rs

@@ -8,3 +8,4 @@
 //! format specific internals, and sync/async I/O workers.
 
 pub mod raw;
+pub mod vhd;