iotests.py: improve verify_image_format helper

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>