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/qcow/mod.rs

@@ -372,8 +372,7 @@ impl Debug for BackingFile {
 /// Parses and validates a QCOW2 image file, returning the metadata, backing
 /// file and sparse flag.
 ///
-/// This shared constructor is used by both QcowFile for sequential I/O
-/// and QcowDiskSync for lock based parallel I/O.
+/// Used by [`QcowFile`] and [`QcowDisk`] constructors.
 pub(crate) fn parse_qcow(
     mut file: RawFile,
     max_nesting_depth: u32,