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/vhdx/mod.rs

@@ -8,6 +8,7 @@ use crate::vhdx::{
     vhdx_io::VhdxIoError,
     vhdx_metadata::{DiskSpec, VhdxMetadataError},
 };
+use crate::BlockBackend;
 use byteorder::{BigEndian, ByteOrder};
 use remain::sorted;
 use std::collections::btree_map::BTreeMap;
@@ -208,6 +209,12 @@ impl Seek for Vhdx {
     }
 }
 
+impl BlockBackend for Vhdx {
+    fn size(&self) -> std::result::Result<u64, crate::Error> {
+        Ok(self.virtual_disk_size())
+    }
+}
+
 impl Clone for Vhdx {
     fn clone(&self) -> Self {
         Vhdx {