block: qcow: impl AsFd for RawFile and QcowRawFile

Implement AsFd for both RawFile and QcowRawFile by delegating to
the inner File handle. This enables safe fd borrowing through the
standard AsFd trait, which is a prerequisite for replacing unsafe
libc::dup calls with BorrowedFd::try_clone_to_owned().

Suggested-by: Rob Bradford <rbradford@rivosinc.com>
Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/qcow/qcow_raw_file.rs

@@ -7,7 +7,7 @@
 use std::fmt::Debug;
 use std::io::{self, BufWriter, Read, Seek, SeekFrom, Write};
 use std::mem::size_of;
-use std::os::fd::{AsRawFd, RawFd};
+use std::os::fd::{AsFd, AsRawFd, BorrowedFd, RawFd};
 
 use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
 use vmm_sys_util::write_zeroes::WriteZeroes;
@@ -361,3 +361,9 @@ impl AsRawFd for QcowRawFile {
         self.file.as_raw_fd()
     }
 }
+
+impl AsFd for QcowRawFile {
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        self.file.as_fd()
+    }
+}

block/src/qcow/raw_file.rs

@@ -11,6 +11,7 @@
 use std::alloc::{Layout, alloc_zeroed, dealloc};
 use std::fs::{File, Metadata};
 use std::io::{self, Read, Seek, SeekFrom, Write};
+use std::os::fd::{AsFd, BorrowedFd};
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::slice;
 
@@ -397,3 +398,9 @@ impl AsRawFd for RawFile {
         self.file.as_raw_fd()
     }
 }
+
+impl AsFd for RawFile {
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        self.file.as_fd()
+    }
+}