qemu-iotests: Fix typo for NFS with IMGOPTSSYNTAX

qemu-iotests: Fix typo for NFS with IMGOPTSSYNTAX

Commit 076003f5 added configuration for NFS with IMGOPTSSYNTAX enabled,
but it didn't use the right variable name: $TEST_DIR_OPTS doesn't exist.
This fixes the mistake.

However, this doesn't make anything work that was broken before: The
only way to get IMGOPTSSYNTAX is with -luks, but the combination of
-luks and -nfs doesn't get qemu-img create commands right (because
qemu-img create doesn't support --image-opts yet), so even after this
fix some more work would be required to make the tests pass.

Reported-by: Tomáš Golembiovský <tgolembi@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>