block_util: Avoid intermediate completion queue allocation

Rather than aggregate the completion list into an intermediate vector
instead adjust the API to provide one completion item at a time.

With DHAT this shows the number of heap allocations has decreased.

Before:

    dhat: Total:     623,852 bytes in 8,157 blocks

After:

    dhat: Total:     380,444 bytes in 3,469 blocks

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

virtio-devices/src/block.rs

@@ -244,8 +244,7 @@ impl BlockEpollHandler {
         let mut read_ops = Wrapping(0);
         let mut write_ops = Wrapping(0);
 
-        let completion_list = self.disk_image.complete();
-        for (user_data, result) in completion_list {
+        while let Some((user_data, result)) = self.disk_image.next_completed_request() {
             let desc_index = user_data as u16;
 
             let mut request = self.find_inflight_request(desc_index)?;