iotests.py: improve verify_image_format helper

Support "generic" formats like in bash tests with their
   _supported_fmt generic
The test, supporting "generic" formats will run if IMGFMT_GENERIC =
true, which is default, except for bochs and cloop. However, you can
use verify_image_format(['generic', 'bochs']), which will run for all
except cloop (for this moment).

Also, add an assert (we don't want set both arguments) and remove
duplication.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/iotests.py

@@ -532,9 +532,17 @@ def notrun(reason):
     sys.exit(0)
 
 def verify_image_format(supported_fmts=[], unsupported_fmts=[]):
-    if supported_fmts and (imgfmt not in supported_fmts):
-        notrun('not suitable for this image format: %s' % imgfmt)
-    if unsupported_fmts and (imgfmt in unsupported_fmts):
+    assert not (supported_fmts and unsupported_fmts)
+
+    if 'generic' in supported_fmts and \
+            os.environ.get('IMGFMT_GENERIC', 'true') == 'true':
+        # similar to
+        #   _supported_fmt generic
+        # for bash tests
+        return
+
+    not_sup = supported_fmts and (imgfmt not in supported_fmts)
+    if not_sup or (imgfmt in unsupported_fmts):
         notrun('not suitable for this image format: %s' % imgfmt)
 
 def verify_platform(supported_oses=['linux']):