iotests: fix race in 030

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 <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Alberto Garcia <[email protected]>
Reviewed-by: Jeff Cody <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>