iotests/118: Create test classes dynamically

iotests/118: Create test classes dynamically

We're getting a ridiculous number of child classes of
TestInitiallyFilled and TestInitiallyEmpty that differ only in a few
attributes that we want to test in all combinations.

Instead of explicitly writing down every combination, let's use a loop
and create those classes dynamically.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>