iotests: rewrite check into python

Just use classes introduced in previous three commits. Behavior
difference is described in these three commits.

Drop group file, as it becomes unused.

Drop common.env: now check is in python, and for tests we use same
python interpreter that runs the check itself. Use build environment
PYTHON in check-block instead, to keep "make check" use the same
python.

Checking for virtio-blk moved to iotests.py, as it actually iotests.py
dependency. Actually not all python iotests depend on it, so in future
it may be refactored to checked only when really needed.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210125185056.129513-6-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>