iotests: improve 169

Before previous patch, iotest 169 was actually broken for the case
test_persistent__not_migbitmap__offline_shared, while formally
passing.

After migration log of vm_b had message:

    qemu-system-x86_64: Could not reopen qcow2 layer: Bitmap already
    exists: bitmap0

which means that invalidation failed and bs->drv = NULL.

It was because we've loaded bitmap twice: on open and on invalidation.

Add code to 169, to catch such fails.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: NJohn Snow <jsnow@redhat.com>

tests/qemu-iotests/169

@@ -24,6 +24,7 @@ import time
 import itertools
 import operator
 import new
+import re
 from iotests import qemu_img
 
 
@@ -133,6 +134,14 @@ class TestDirtyBitmapMigration(iotests.QMPTestCase):
 
         if should_migrate:
             self.vm_b.shutdown()
+
+            # catch 'Could not reopen qcow2 layer: Bitmap already exists'
+            # possible error
+            log = self.vm_b.get_log()
+            log = re.sub(r'^\[I \d+\.\d+\] OPENED\n', '', log)
+            log = re.sub(r'\[I \+\d+\.\d+\] CLOSED\n?$', '', log)
+            self.assertEqual(log, '')
+
             # recreate vm_b, as we don't want -incoming option (this will lead
             # to "cat" process left alive after test finish)
             self.vm_b = iotests.VM(path_suffix='b')