block: batch submit requests for fixed VHD

Updated VHD async implementation to call the batch submit
method via the raw async IO layer.

Signed-off-by: Muminul Islam <muislam@microsoft.com>

block/src/fixed_vhd_async.rs

@@ -12,7 +12,7 @@ use crate::async_io::{
 };
 use crate::fixed_vhd::FixedVhd;
 use crate::raw_async::RawFileAsync;
-use crate::BlockBackend;
+use crate::{BatchRequest, BlockBackend};
 
 pub struct FixedVhdDiskAsync(FixedVhd);
 
@@ -106,4 +106,12 @@ impl AsyncIo for FixedVhdAsync {
     fn next_completed_request(&mut self) -> Option<(u64, i32)> {
         self.raw_file_async.next_completed_request()
     }
+
+    fn batch_requests_enabled(&self) -> bool {
+        true
+    }
+
+    fn submit_batch_requests(&mut self, batch_request: &[BatchRequest]) -> AsyncIoResult<()> {
+        self.raw_file_async.submit_batch_requests(batch_request)
+    }
 }