iotests: Use case_skip() in skip_if_unsupported()

iotests: Use case_skip() in skip_if_unsupported()

skip_if_unsupported() should use the stronger variant case_skip(),
because this allows it to be used even with setUp() (in a meaningful
way).

In the process, make it explicit what we expect the first argument of
the func_wrapper to be (namely something derived of QMPTestCase).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190917092004.999-4-mreitz@redhat.com
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>