sd: make preallocate optional in volume validation

In the current effort to move to the new storage API the preallocate
option may be omitted and therefore should not be validated.

Relates-To: http://bugzilla.redhat.com/show_bug.cgi?id=1097843
Change-Id: Ide89098b404e992678441d4b6f996ff203e055f0
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28129
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>