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/mod.rs

@@ -5,6 +5,7 @@
 use std::collections::btree_map::BTreeMap;
 use std::fs::File;
 use std::io::{Read, Seek, SeekFrom, Write};
+use std::os::fd::{AsRawFd, RawFd};
 
 use byteorder::{BigEndian, ByteOrder};
 use remain::sorted;
@@ -222,6 +223,12 @@ impl Clone for Vhdx {
     }
 }
 
+impl AsRawFd for Vhdx {
+    fn as_raw_fd(&self) -> RawFd {
+        self.file.as_raw_fd()
+    }
+}
+
 pub(crate) fn uuid_from_guid(buf: &[u8]) -> Uuid {
     // The first 3 fields of UUID are stored in Big Endian format, and
     // the last 8 bytes are stored as byte array. Therefore, we read the