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/raw_sync.rs

@@ -43,6 +43,10 @@ impl DiskFile for RawFileDiskSync {
             DiskTopology::default()
         }
     }
+
+    fn fd(&mut self) -> RawFd {
+        self.file.as_raw_fd()
+    }
 }
 
 pub struct RawFileSync {