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>

vmm/src/device_manager.rs

@@ -937,7 +937,7 @@ impl DeviceManager {
                     .map_err(DeviceManagerError::DetectImageType)?;
                 match image_type {
                     ImageType::Raw => {
-                        let raw_img = vm_virtio::RawFile::new(raw_img);
+                        let raw_img = vm_virtio::RawFile::new(raw_img, false);
                         let dev = vm_virtio::Block::new(
                             raw_img,
                             disk_cfg.path.clone(),