block: Implement FileExt for RawFile

Implement std::os::unix::fs::FileExt for RawFile by delegating to the
inner File. This enables callers holding a reference to a RawFile to use
read_exact_at and write_all_at directly for non-cursor I/O (like pread,
etc) without going through custom helper functions.

Assisted-by: Claude:Opus-4.6
Signed-off-by: Rob Bradford <rbradford@meta.com>

block/src/formats/qcow/internal/raw_file.rs

@@ -12,6 +12,7 @@ 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::fs::FileExt;
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::{result, slice};
 
@@ -404,3 +405,13 @@ impl AsFd for RawFile {
         self.file.as_fd()
     }
 }
+
+impl FileExt for RawFile {
+    fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        self.file.read_at(buf, offset)
+    }
+
+    fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        self.file.write_at(buf, offset)
+    }
+}