qemu-iotests: mark 035 qcow2-only

The 035 parallel aio write test relies on knowledge of qcow2 metadata
layout to stress parallel L2 table accesses.  This only works for qcow2
unless we add additional calculations for qed or other formats.

Mark this test as qcow2-only.

Note that the test is strictly speaking non-deterministic although the
output produced is reliable with qcow2.  This is because the aio_write
command returns before the aio write request has completed.  Completions
can occur at any time afterwards and cause a message to be printed.
Therefore the exact output of this test is not deterministic but we seem
to get away with it for qcow2 (maybe due to coroutine and main loop
scheduling).

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
