fix(block): make prepare_save() more robust

If prepare_save() fails to drain the io_uring queues (when used) and
sync the host filesystem we might end up with a corrupted disk snapshot.
Currently, Firecracker ignores that, only emitting an error message.

Be more strict and expect no errors, so that we can have a better
post-mortem analysis of what happened.

Signed-off-by: Babis Chalios <babis.chalios@e2b.dev>

Author: bchalios

src/vmm/src/devices/virtio/block/virtio/device.rs

@@ -197,7 +197,7 @@ pub struct ConfigSpace {
     pub max_write_zeroes_seg: u32,       // offset 52
     pub write_zeroes_may_unmap: u8,      // offset 56
     pub(crate) _unused1: [u8; 3],        // offset 57 (spec field — virtio_blk_config.unused1)
-    pub(crate) _pad: [u8; 4],            // offset 60 (Rust alignment padding to 64; spec ends at 60)
+    pub(crate) _pad: [u8; 4], // offset 60 (Rust alignment padding to 64; spec ends at 60)
 }
 const _: () = assert!(std::mem::size_of::<ConfigSpace>() == 64);
 // Compile-time guards against accidental layout drift. The byte offsets here
@@ -722,9 +722,10 @@ impl VirtioBlock {
     }
 
     fn drain_and_flush(&mut self, discard: bool) {
-        if let Err(err) = self.disk.file_engine.drain_and_flush(discard) {
-            error!("Failed to drain ops and flush block data: {:?}", err);
-        }
+        self.disk
+            .file_engine
+            .drain_and_flush(discard)
+            .expect("virtio-block: failed to drain ops and flush block data");
     }
 
     /// Prepare device for being snapshotted.