iotests: add dirty bitmap postcopy test

iotests: add dirty bitmap postcopy test

Test
- start two vms (vm_a, vm_b)

- in a
    - do writes from set A
    - do writes from set B
    - fix bitmap sha256
    - clear bitmap
    - do writes from set A
    - start migration
- than, in b
    - wait vm start (postcopy should start)
    - do writes from set B
    - check bitmap sha256

The test should verify postcopy migration and then merging with delta
(changes in target, during postcopy process).

Reduce supported cache modes to only 'none', because with cache on time
from source.STOP to target.RESUME is unpredictable and we can fail with
timout while waiting for target.RESUME.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20180313180320.339796-14-vsementsov@virtuozzo.com