iotests: replace mutable list default args

iotests: replace mutable list default args

It's bad hygiene: if we modify this list, it will be modified across all
invocations.

(Remaining bad usages are fixed in a subsequent patch which changes the
function signature anyway.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200331000014.11581-5-jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>