iotests: Make 030 less flaky

This patch fixes two race conditions in 030:

1. The first is in TestENOSPC.test_enospc().  After resuming the job,
   querying it to confirm it is no longer paused may fail because in the
   meantime it might have completed already.  The same was fixed in
   TestEIO.test_ignore() already (in commit
   2c3b44da).

2. The second is in TestSetSpeed.test_set_speed_invalid(): Here, a
   stream job is started on a drive without any break points, with a
   block-job-set-speed invoked subsequently.  However, without any break
   points, the job might have completed in the meantime (on tmpfs at
   least); or it might complete before cancel_and_wait() which expects
   the job to still exist.  This can be fixed like everywhere else by
   pausing the drive (installing break points) before starting the job
   and letting cancel_and_wait() resume it.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20171109203025.27493-2-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>