nimmanager: always read all xml files (if present)

At the point where enigma decides to either do or don't
read an xml file (satellites.xml/terrestrial.xml/cables.xml),
no information is yet known about multi-type tuners, so some
xml are skipped while they shouldn't.

For the moment workaround this by always reading all xml file
(if present), as this probably is a chicken-egg-problem.