qemu-iotests: Reduce racy output in 028

On my machine, './check -qcow2 028' was failing about 80% of the
time, due to a race in how many times the repeated attempts
to run 'info block-jobs' could occur before the job was done,
showing up as a failure of fewer '(qemu) ' prompts than in the
expected output.  Silence the output during the repetitions, then
add a final clean command to keep the expected output useful;
once patched, I was finally able to run the test 20 times in a
row with no failures.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
