virstorageobj: Move virStoragePoolObjSourceFindDuplicate and friends up

This function is going to be made static in used in
virStoragePoolObjAssignDef(). Therefore move it and all the
static functions it calls a few lines up.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>