tests/qemu-iotests/tests/mirror-sparse: actually require O_DIRECT

tests/qemu-iotests/tests/mirror-sparse: actually require O_DIRECT

Commit c0ddcb2cbc146e introduced the test which uses cache=direct
mode, without checking if the scratch filesystem supports O_DIRECT.
A subsequent commit, afeb002e0ad49d, tried to fix that issue, but
instead of checking for o_direct, it checked for
`_supported_cache_modes none directsync`, which is not what the
original mirror-sparse test uses.  Fix both by actually checking
for o_direct.

Fixes: c0ddcb2cbc146e "tests: Add iotest mirror-sparse for recent patches"
Fixes: afeb002e0ad49d "tests/qemu-iotests/tests/mirror-sparse: skip if O_DIRECT is not supported"
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>