block/backup: use qemu_iovec_init_buf

Use new qemu_iovec_init_buf() instead of
qemu_iovec_init_external( ... , 1), which simplifies the code.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190218140926.333779-5-vsementsov@virtuozzo.com
Message-Id: <20190218140926.333779-5-vsementsov@virtuozzo.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/backup.c

@@ -107,7 +107,6 @@ static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
                                                       void **bounce_buffer)
 {
     int ret;
-    struct iovec iov;
     QEMUIOVector qiov;
     BlockBackend *blk = job->common.blk;
     int nbytes;
@@ -119,9 +118,7 @@ static int coroutine_fn backup_cow_with_bounce_buffer(BackupBlockJob *job,
     if (!*bounce_buffer) {
         *bounce_buffer = blk_blockalign(blk, job->cluster_size);
     }
-    iov.iov_base = *bounce_buffer;
-    iov.iov_len = nbytes;
-    qemu_iovec_init_external(&qiov, &iov, 1);
+    qemu_iovec_init_buf(&qiov, *bounce_buffer, nbytes);
 
     ret = blk_co_preadv(blk, start, qiov.size, &qiov, read_flags);
     if (ret < 0) {