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/vhd/worker/mod.rs

@@ -0,0 +1,12 @@
+// Copyright 2026 The Cloud Hypervisor Authors. All rights reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+
+//! Sync/async I/O workers for fixed VHD images.
+//!
+//! Thin wrappers around the raw workers that clamp I/O to the
+//! virtual disk size.
+
+#[cfg(feature = "io_uring")]
+pub(crate) mod async_uring;
+pub(crate) mod sync;