rbd: Use RBD format 2 by default when creating images.

rbd: Use RBD format 2 by default when creating images.

We used to look at the librbd code version and depending on that
we would invoke rbd_create3() or rbd_create().

Since librbd version 0.67.9 we can however tell RBD that it should
create rbd format 2 images even if we invoke rbd_create().

The less options we pass to librbd, the more we can lean on the sane
defaults it uses.

For rbd_create3() we had things like the stripe count and unit hardcoded
in libvirt and that might cause problems down the road.

Signed-off-by: Wido den Hollander <wido@widodh.nl>