vm-virtio: block: Add support for alignment restrictions

Doing I/O on an image opened with O_DIRECT requires to adhere to
certain restrictions, requiring the following elements to be aligned:

 - Address of the source/destination memory buffer.
 - File offset.
 - Length of the data to be read/written.

The actual alignment value depends on various elements, and according
to open(2) "(...) there is currently no filesystem-independent
interface for an application to discover these restrictions (...)".

To discover such value, we iterate through a list of alignments
(currently, 512 and 4096) calling pread() with each one and checking
if the operation succeeded.

We also extend RawFile so it can be used as a backend for QcowFile,
so the later can be easily adapted to support O_DIRECT too.

Signed-off-by: Sergio Lopez <slp@redhat.com>

src/bin/vhost_user_blk.rs

@@ -87,7 +87,9 @@ impl VhostUserBlkBackend {
         let image_id = build_disk_image_id(&PathBuf::from(&image_path));
         let image_type = qcow::detect_image_type(&raw_img).unwrap();
         let mut image = match image_type {
-            ImageType::Raw => Box::new(vm_virtio::RawFile::new(raw_img)) as Box<dyn DiskFile>,
+            ImageType::Raw => {
+                Box::new(vm_virtio::RawFile::new(raw_img, false)) as Box<dyn DiskFile>
+            }
             ImageType::Qcow2 => Box::new(QcowFile::from(raw_img).unwrap()) as Box<dyn DiskFile>,
         };