One shot prepare.

The number of storage accesses is reduced to 1 or none instead of
the recursive volume images produces.
Requires a new selinux policy.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=960952
Related to BZ#769502, BZ#990509.

Change-Id: Id47db9c53199385dd5d08586e4782ea23885eb72
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/4220
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>