block: Use logical block size for alignment

O_DIRECT requires buffer addresses to be aligned to the backend
device's logical block size. The existing bounce buffer logic in
execute_async() hardcodes SECTOR_SIZE (512) for the alignment check
and bounce buffer allocation. This is insufficient for devices with
a 4096-byte logical block size, where misaligned buffers cause
-EINVAL from the host kernel.

Add an alignment() method to the AsyncIo trait that returns the
backend's logical block size, defaulting to SECTOR_SIZE. The three
raw I/O backends (io_uring, AIO, synchronous) probe the device
topology via DiskTopology::probe() at creation time and return the
actual logical block size. All image format backends would simply
use the default value of 512 bytes since their underlying are
not block devices.

execute_async() now queries disk_image.alignment() instead of using
the hardcoded SECTOR_SIZE

Fixes: #7720

Signed-off-by: Saravanan D <saravanand@crusoe.ai>

block/src/async_io.rs

@@ -8,7 +8,7 @@ use std::os::fd::{AsRawFd, OwnedFd, RawFd};
 use thiserror::Error;
 use vmm_sys_util::eventfd::EventFd;
 
-use crate::{BatchRequest, DiskTopology};
+use crate::{BatchRequest, DiskTopology, SECTOR_SIZE};
 
 #[derive(Error, Debug)]
 pub enum DiskFileError {
@@ -145,4 +145,7 @@ pub trait AsyncIo: Send {
     fn submit_batch_requests(&mut self, _batch_request: &[BatchRequest]) -> AsyncIoResult<()> {
         Ok(())
     }
+    fn alignment(&self) -> u64 {
+        SECTOR_SIZE
+    }
 }