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/fixed_vhd_sync.rs

@@ -33,6 +33,10 @@ impl DiskFile for FixedVhdDiskSync {
                 .map_err(DiskFileError::NewAsyncIo)?,
         ) as Box<dyn AsyncIo>)
     }
+
+    fn fd(&mut self) -> RawFd {
+        self.0.as_raw_fd()
+    }
 }
 
 pub struct FixedVhdSync {