Storage: Check that underlying file system supports direct IO

- Check when creating a domain
- Check when creating a domain object

This is done so that we don't allow posixfs domains on top of targets
that don't support direct IO.

Direct IO is needed in order to read the metadata reliably in a
clustered environment. It is also a required feature for Sanlock.

Change-Id: Icf14d1c4737a88e693e5bebb896aef382b8b424c
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/9749
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>