block: Add disk_file module skeleton

Composable disk capability traits with DiskFile as a supertrait
bundling DiskSize and Geometry. Optional capabilities are
separate traits: PhysicalSize, DiskFd, SparseCapable, Resizable.
AsyncDiskFile extends DiskFile with async I/O construction.

Empty module with doc comment, trait definitions follow.

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

block/src/lib.rs

@@ -9,6 +9,7 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
 pub mod async_io;
+pub mod disk_file;
 pub mod error;
 pub mod fcntl;
 pub mod fixed_vhd;