block: qcow: Add unified QcowDisk wrapper

Introduce QcowDisk, a unified DiskFile implementation for QCOW2
disk images that handles backend selection at runtime via a
use_io_uring flag, matching the pattern used by FixedVhdDisk.

The wrapper delegates to QcowSync or QcowAsync based on the flag
and includes a compile time guard that returns an error when
io_uring is requested but the feature is not enabled.

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

block/src/lib.rs

@@ -23,6 +23,7 @@ pub mod qcow;
 #[cfg(feature = "io_uring")]
 pub mod qcow_async;
 pub(crate) mod qcow_common;
+pub mod qcow_disk;
 pub mod qcow_sync;
 #[cfg(feature = "io_uring")]
 pub(crate) mod raw_async;