iotests: improve pause_job

It's possible, that job was finished during waiting. In this case we
will see error message "Timeout waiting for job to pause" which is not
very informative. So, let's check during waiting iteration that the job
exists.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20180601115923.17159-1-vsementsov@virtuozzo.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/iotests.py

@@ -590,9 +590,14 @@ class QMPTestCase(unittest.TestCase):
         with Timeout(1, "Timeout waiting for job to pause"):
             while True:
                 result = self.vm.qmp('query-block-jobs')
+                found = False
                 for job in result['return']:
-                    if job['device'] == job_id and job['paused'] == True and job['busy'] == False:
-                        return job
+                    if job['device'] == job_id:
+                        found = True
+                        if job['paused'] == True and job['busy'] == False:
+                            return job
+                        break
+                assert found
 
     def pause_job(self, job_id='job0', wait=True):
         result = self.vm.qmp('block-job-pause', device=job_id)