block: Implement DiskFd trait for RawFileDiskAio

Add disk_file::DiskFd trait implementation for
RawFileDiskAio. Delegates to file.as_raw_fd() via
BorrowedDiskFd, taking &self instead of &mut self.

Signed-off-by: Muminul Islam <muislam@microsoft.com>

block/src/raw_async_aio.rs

@@ -86,6 +86,12 @@ impl disk_file::PhysicalSize for RawFileDiskAio {
     }
 }
 
+impl disk_file::DiskFd for RawFileDiskAio {
+    fn fd(&self) -> BorrowedDiskFd<'_> {
+        BorrowedDiskFd::new(self.file.as_raw_fd())
+    }
+}
+
 pub struct RawFileAsyncAio {
     fd: RawFd,
     ctx: aio::IoContext,