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: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: John Snow <jsnow@redhat.com>