layer = iface.activeLayer()
iter = layer.getFeatures()
for feature in iter:
    # retrieve every feature with its geometry and attributes
    # fetch geometry
    geom = feature.geometry()
    # assume z values are stored in fields "from" and "to"
    p0z = feature["from"]
    p1z = feature["to"]

    # access Points, assuming each line has to vertices
    oldp0, oldp1 = geom.asPolyline()[0], geom.asPolyline()[1]

    # creating a new LineString with PointZ-vertices
    newp0 = QgsPointV2( QgsWKBTypes.PointZ, oldp0[0], oldp0[1], p0z )
    newp1 = QgsPointV2( QgsWKBTypes.PointZ, oldp1[0], oldp1[1], p1z )
    linez = QgsLineStringV2()
    linez.setPoints([newp0, newp1])
    # output
    print linez.asWkt()