block: Move raw format files into formats/raw/

Move raw format implementation into a structured directory layout:

  raw_disk.rs        -> formats/raw/mod.rs                (RawDisk)
  raw_sync.rs        -> formats/raw/worker/sync.rs        (RawSync)
  raw_async.rs       -> formats/raw/worker/async_uring.rs (RawAsync)
  raw_async_aio.rs   -> formats/raw/worker/async_aio.rs   (RawAio)
  raw_async_io_tests.rs -> formats/raw/worker/tests.rs

Update imports in fixed_vhd_sync.rs and fixed_vhd_async.rs to use
the new paths. Re-export formats::raw as raw_disk in lib.rs to
preserve the external API.

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

block/src/formats/mod.rs

@@ -0,0 +1,10 @@
+// Copyright 2026 The Cloud Hypervisor Authors. All rights reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+
+//! Disk format implementations.
+//!
+//! Each format lives in its own submodule with a `DiskFile` wrapper,
+//! format specific internals, and sync/async I/O workers.
+
+pub mod raw;