api: expose prepare and teardown image

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1092166
Change-Id: I6a4e21513a2a888ff5ee4bc085a3da9a8bf8980b
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28389
Reviewed-by: Jiří Moskovčák <jmoskovc@redhat.com>
Tested-by: Jiří Moskovčák <jmoskovc@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>