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_sync.rs

@@ -7,6 +7,7 @@ use crate::async_io::{
 };
 use crate::fixed_vhd::FixedVhd;
 use crate::raw_sync::RawFileSync;
+use crate::BlockBackend;
 use std::fs::File;
 use std::os::unix::io::{AsRawFd, RawFd};
 use vmm_sys_util::eventfd::EventFd;
@@ -21,12 +22,12 @@ impl FixedVhdDiskSync {
 
 impl DiskFile for FixedVhdDiskSync {
     fn size(&mut self) -> DiskFileResult<u64> {
-        Ok(self.0.size())
+        Ok(self.0.size().unwrap())
     }
 
     fn new_async_io(&self, _ring_depth: u32) -> DiskFileResult<Box<dyn AsyncIo>> {
         Ok(Box::new(
-            FixedVhdSync::new(self.0.as_raw_fd(), self.0.size())
+            FixedVhdSync::new(self.0.as_raw_fd(), self.0.size().unwrap())
                 .map_err(DiskFileError::NewAsyncIo)?,
         ) as Box<dyn AsyncIo>)
     }