block: disk_file: Add DiskFd trait

Backing file descriptor access for disk images backed by
a file. Returns a BorrowedDiskFd that wraps the raw fd
with lifetime tracking. Not available for network or
memory backed disk formats.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/disk_file.rs

@@ -36,6 +36,7 @@
 use std::fmt::Debug;
 
 use crate::BlockResult;
+use crate::async_io::BorrowedDiskFd;
 
 /// Reported capacity of a disk image.
 pub trait DiskSize: Send + Debug {
@@ -48,3 +49,9 @@ pub trait PhysicalSize: Send + Debug {
     /// Actual bytes occupied on the host filesystem.
     fn physical_size(&self) -> BlockResult<u64>;
 }
+
+/// Backing file descriptor access for disk images backed by a file.
+pub trait DiskFd: Send + Debug {
+    /// Borrows the underlying file descriptor.
+    fn fd(&self) -> BorrowedDiskFd<'_>;
+}