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>

tests/qemu-iotests/028

@@ -114,10 +114,12 @@ h=$QEMU_HANDLE
 QEMU_COMM_TIMEOUT=1
 
 # Silence output since it contains the disk image path and QEMU's readline
-# character echoing makes it very hard to filter the output
+# character echoing makes it very hard to filter the output. Plus, there
+# is no telling how many times the command will repeat before succeeding.
 _send_qemu_cmd $h "drive_backup disk ${TEST_IMG}.copy" "(qemu)" >/dev/null
 _send_qemu_cmd $h "" "Formatting" | _filter_img_create
-qemu_cmd_repeat=20 _send_qemu_cmd $h "info block-jobs" "No active jobs"
+qemu_cmd_repeat=20 _send_qemu_cmd $h "info block-jobs" "No active jobs" >/dev/null
+_send_qemu_cmd $h "info block-jobs" "No active jobs"
 _send_qemu_cmd $h 'quit' ""
 
 # Base image sectors

tests/qemu-iotests/028.out

@@ -469,10 +469,7 @@ No errors were found on the image.
 block-backup
 
 Formatting 'TEST_DIR/t.IMGFMT.copy', fmt=IMGFMT size=4294968832 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
-(qemu)
 (qemu) iininfinfoinfo info binfo blinfo bloinfo blocinfo blockinfo block-info block-jinfo block-joinfo block-jobinfo block-jobs
-Type backup, device disk: Completed 0 of 4294968832 bytes, speed limit 0 bytes/s
-iininfinfoinfo info binfo blinfo bloinfo blocinfo blockinfo block-info block-jinfo block-joinfo block-jobinfo block-jobs
 No active jobs
 === IO: pattern 195
 read 512/512 bytes at offset 3221194240