block: Create io/ module for shared I/O infrastructure

Move async_io.rs, fcntl.rs, and request.rs into block/src/io/. These
files are generic I/O infrastructure shared by all formats rather
than format specific code.

The io/ directory name clashes with std::io in lib.rs, so the module
is declared as io_impl via #[path] and the submodules are re-exported
at the crate root to keep existing import paths working.

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

block/src/io/mod.rs

@@ -0,0 +1,12 @@
+// Copyright 2026 The Cloud Hypervisor Authors. All rights reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+
+//! Shared I/O infrastructure for all disk format backends.
+//!
+//! Contains the async I/O trait, request handling, and file locking
+//! helpers.
+
+pub mod async_io;
+pub mod fcntl;
+pub mod request;