block_stream_reboot: add random sleep time

give random sleep time for sub-test initialization
Signed-off-by: NXu Tian <xutian@redhat.com>

qemu/tests/block_stream_reboot.py

+import time
+import random
 from autotest.client.shared import error, utils
 from qemu.tests import blk_stream
 
@@ -30,6 +32,7 @@ class BlockStreamReboot(blk_stream.BlockStream):
                 bg.start()
                 if bg.isAlive():
                     self.process.append(bg)
+            time.sleep(random.randint(25, 100))
 
     def clean(self):
         """