iotests: Test unaligned raw images with O_DIRECT

iotests: Test unaligned raw images with O_DIRECT

We already have 221 for accesses through the page cache, but it is
better to create a new file for O_DIRECT instead of integrating those
test cases into 221.  This way, we can make use of
_supported_cache_modes (and _default_cache_mode) so the test is
automatically skipped on filesystems that do not support O_DIRECT.

As part of the split, add _supported_cache_modes to 221.  With that, it
no longer fails when run with -c none or -c directsync.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>