blockdev: Fix NULL pointer dereference in blockdev-add

If aio=native, we check that cache.direct is set as well. If however
cache wasn't specified at all, qemu just segfaulted.

The old condition didn't make any sense anyway because it effectively
only checked for the default cache mode case, but not for an explicitly
set cache.direct=off mode.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Eric Blake <eblake@redhat.com>
