block: enable to get a raw FD of each block device's DiskFile

This is a prerequisite for the next steps.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

block/src/qcow_sync.rs

@@ -5,6 +5,7 @@
 use std::collections::VecDeque;
 use std::fs::File;
 use std::io::{Seek, SeekFrom};
+use std::os::fd::{AsRawFd, RawFd};
 use std::sync::{Arc, Mutex, MutexGuard};
 
 use vmm_sys_util::eventfd::EventFd;
@@ -35,6 +36,11 @@ impl DiskFile for QcowDiskSync {
     fn new_async_io(&self, _ring_depth: u32) -> DiskFileResult<Box<dyn AsyncIo>> {
         Ok(Box::new(QcowSync::new(self.qcow_file.clone())) as Box<dyn AsyncIo>)
     }
+
+    fn fd(&mut self) -> RawFd {
+        let lock = self.qcow_file.lock().unwrap();
+        lock.as_raw_fd()
+    }
 }
 
 pub struct QcowSync {