iotests.py: implement unsupported_imgopts

iotests.py: implement unsupported_imgopts

We are going to support some addition IMGOPTS in python iotests like
in bash iotests. Similarly to bash iotests, we want a way to skip some
tests which can't work with specific IMGOPTS.

Globally for python iotests we will not support things like
'data_file=$TEST_IMG.ext_data_file' in IMGOPTS, so, forbid this
globally in iotests.py.

Suggested-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211223160144.1097696-3-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>