block: introduce trait BlockBackend for block types

This commit introduces the trait `BlockBackend` with generic ops
including read, write and seek, which can be used for common I/O
interfaces for the block types without using `DiskFile` and `AsyncIo`.

Signed-off-by: Yu Li <liyu.yukiteru@bytedance.com>

block/src/fixed_vhd.rs

@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use crate::vhd::VhdFooter;
+use crate::BlockBackend;
 use std::fs::File;
 use std::io::{Read, Seek, SeekFrom, Write};
 use std::os::unix::io::{AsRawFd, RawFd};
@@ -24,10 +25,6 @@ impl FixedVhd {
             position: 0,
         })
     }
-
-    pub fn size(&self) -> u64 {
-        self.size
-    }
 }
 
 impl AsRawFd for FixedVhd {
@@ -76,6 +73,12 @@ impl Seek for FixedVhd {
     }
 }
 
+impl BlockBackend for FixedVhd {
+    fn size(&self) -> std::result::Result<u64, crate::Error> {
+        Ok(self.size)
+    }
+}
+
 impl Clone for FixedVhd {
     fn clone(&self) -> Self {
         Self {