block: raw: Add RawDisk struct with RawBackend enum

Introduce the unified DiskFile wrapper for raw disk images. The
RawBackend enum selects between sync, io_uring, and AIO backends
at construction time.

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

block/src/lib.rs

@@ -31,6 +31,7 @@ pub mod raw_async;
 pub mod raw_async_aio;
 #[cfg(test)]
 mod raw_async_io_tests;
+pub mod raw_disk;
 pub mod raw_sync;
 mod request;
 pub mod vhd;