iotests: fix race in 030

the stop_test case tests that we can resume a block-stream
command after it has stopped/paused due to error. We cannot
always reliably query it before it finishes after resume, though,
so make this a conditional.

The important thing is that we are still testing that it has stopped,
and that it finishes successfully after we send a resume command.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
