qemu-iotests: Fix supported cache modes for 052

qemu-iotests: Fix supported cache modes for 052

The requirement for this test case is really "no O_DIRECT", because the
temporary snapshot for BDRV_O_SNAPSHOT is created in /tmp, which often
is a tmpfs.

Commit f210a83c ('qemu-iotests: Add _default_cache_mode and
_supported_cache_modes') turned the restriction into writethrough-only,
but that's not really necessary.

Allow to run the test for any non-O_DIRECT cache modes, and use the
global default of writeback if no cache mode is specified.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1412076430-11623-3-git-send-email-kwolf@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>