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/qcow_sync.rs

@@ -165,7 +165,7 @@ pub struct QcowSync {
 }
 
 impl QcowSync {
-    fn new(
+    pub(crate) fn new(
         metadata: Arc<QcowMetadata>,
         data_file: QcowRawFile,
         backing_file: Option<Arc<dyn BackingRead>>,