block: qcow: Remove old wrapper structs, restrict visibility

Delete QcowDiskSync and QcowDiskAsync wrapper structs along with
their DiskFile trait impls. Only the AsyncIo worker structs
QcowSync and QcowAsync remain. Reduce module visibility of
qcow_sync and qcow_async to pub(crate).

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

block/src/lib.rs

@@ -21,10 +21,10 @@ pub mod fixed_vhd_disk;
 pub mod fixed_vhd_sync;
 pub mod qcow;
 #[cfg(feature = "io_uring")]
-pub mod qcow_async;
+pub(crate) mod qcow_async;
 pub(crate) mod qcow_common;
 pub mod qcow_disk;
-pub mod qcow_sync;
+pub(crate) mod qcow_sync;
 #[cfg(feature = "io_uring")]
 pub(crate) mod raw_async;
 pub(crate) mod raw_async_aio;