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

tests/qemu-iotests/iotests.py

@@ -504,6 +504,10 @@ def verify_platform(supported_oses=['linux']):
     if True not in [sys.platform.startswith(x) for x in supported_oses]:
         notrun('not suitable for this OS: %s' % sys.platform)
 
+def verify_cache_mode(supported_cache_modes=[]):
+    if supported_cache_modes and (cachemode not in supported_cache_modes):
+        notrun('not suitable for this cache mode: %s' % cachemode)
+
 def supports_quorum():
     return 'quorum' in qemu_img_pipe('--help')
 
@@ -512,7 +516,7 @@ def verify_quorum():
     if not supports_quorum():
         notrun('quorum support missing')
 
-def main(supported_fmts=[], supported_oses=['linux']):
+def main(supported_fmts=[], supported_oses=['linux'], supported_cache_modes=[]):
     '''Run tests'''
 
     global debug
@@ -529,6 +533,7 @@ def main(supported_fmts=[], supported_oses=['linux']):
     verbosity = 1
     verify_image_format(supported_fmts)
     verify_platform(supported_oses)
+    verify_cache_mode(supported_cache_modes)
 
     # We need to filter out the time taken from the output so that qemu-iotest
     # can reliably diff the results against master output.