block: Add "drained begin/end" for transactional backup

This ensures the atomicity of the transaction by avoiding processing of
external requests such as those from ioeventfd.

Move the assignment to state->bs up right after bdrv_drained_begin, so
that we can use it in the clean callback. The abort callback will still
check bs->job and state->job, so it's OK.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -1684,9 +1684,16 @@ static void drive_backup_prepare(BlkTransactionState *common, Error **errp)
         return;
     }
 
+    if (!blk_is_available(blk)) {
+        error_setg(errp, QERR_DEVICE_HAS_NO_MEDIUM, backup->device);
+        return;
+    }
+
     /* AioContext is released in .clean() */
     state->aio_context = blk_get_aio_context(blk);
     aio_context_acquire(state->aio_context);
+    bdrv_drained_begin(blk_bs(blk));
+    state->bs = blk_bs(blk);
 
     qmp_drive_backup(backup->device, backup->target,
                      backup->has_format, backup->format,
@@ -1702,7 +1709,6 @@ static void drive_backup_prepare(BlkTransactionState *common, Error **errp)
         return;
     }
 
-    state->bs = blk_bs(blk);
     state->job = state->bs->job;
 }
 
@@ -1722,6 +1728,7 @@ static void drive_backup_clean(BlkTransactionState *common)
     DriveBackupState *state = DO_UPCAST(DriveBackupState, common, common);
 
     if (state->aio_context) {
+        bdrv_drained_end(state->bs);
         aio_context_release(state->aio_context);
     }
 }