New getChildrenList implementation.

This implementation returns children of any image on the SD.
Avoids to produce each volume in the domain.

Change-Id: I584cd5d1b03d3965457f12c3d67de95455d1de24
Related-to: https://bugzilla.redhat.com/show_bug.cgi?id=960952
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15765
Reviewed-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>