iotests: extended timeout under Valgrind

As the iotests run longer under the Valgrind, the QEMU_COMM_TIMEOUT is
to be increased in the test cases 028, 183 and 192 when running under
the Valgrind.

Suggested-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>