qemu-iotests: fix 185

qemu-iotests: fix 185

185 can sometimes produce wrong output like this:

    185 2s ... - output mismatch (see 185.out.bad)
    --- /work/src/qemu/master/tests/qemu-iotests/185.out    2017-07-14 \
        15:14:29.520343805 +0300
    +++ 185.out.bad 2017-08-07 16:51:02.231922900 +0300
    @@ -37,7 +37,7 @@
     {"return": {}}
     {"return": {}}
     {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, \
         "event": "SHUTDOWN", "data": {"guest": false}}
    -{"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, \
        "event": "BLOCK_JOB_CANCELLED", "data": {"device": "disk", \
            "len": 4194304, "offset": 4194304, "speed": 65536, "type": \
                "mirror"}}
    +{"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, \
        "event": "BLOCK_JOB_CANCELLED", "data": {"device": "disk", \
            "len": 0, "offset": 0, "speed": 65536, "type": "mirror"}}

     === Start backup job and exit qemu ===

    Failures: 185
    Failed 1 of 1 tests

This is because, under heavy load, the quit can happen before the first
iteration of the mirror request has occurred.  To make sure we've had
time to iterate, let's just add a sleep for 0.5 seconds before quitting.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>