iotests: fix 169

iotests: fix 169

Improve and fix 169:
    - use MIGRATION events instead of RESUME
    - make a TODO: enable dirty-bitmaps capability for offline case
    - recreate vm_b without -incoming near test end

This (likely) fixes racy faults at least of the following types:

    - timeout on waiting for RESUME event
    - sha256 mismatch on line 136 (142 after this patch)
    - fail to self.vm_b.launch() on line 135 (141 now after this patch)

And surely fixes cat processes, left after test finish.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20180411122606.367301-3-vsementsov@virtuozzo.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>