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/vhdx_sync.rs

@@ -4,6 +4,7 @@
 
 use std::collections::VecDeque;
 use std::fs::File;
+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 VhdxDiskSync {
                 as Box<dyn AsyncIo>,
         )
     }
+
+    fn fd(&mut self) -> RawFd {
+        let lock = self.vhdx_file.lock().unwrap();
+        lock.as_raw_fd()
+    }
 }
 
 pub struct VhdxSync {