block: qcow_async: Add QcowAsync struct and constructor

Per queue I/O worker that uses io_uring for asynchronous reads
against fully allocated clusters. The struct holds the shared
metadata, data file, optional backing reader, the io_uring
instance and a synthetic completion list.

Feature gated on io_uring in lib.rs.

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

block/src/lib.rs

@@ -18,6 +18,7 @@ pub mod fixed_vhd;
 pub mod fixed_vhd_async;
 pub mod fixed_vhd_sync;
 pub mod qcow;
+#[cfg(feature = "io_uring")]
 pub mod qcow_async;
 pub(crate) mod qcow_common;
 pub mod qcow_sync;