Change ComputerDriver.legacy_nwinfo to raise by default.

This makes non-updated hypervisors visible by grepping for 'def
legacy_nwinfo' rather than only the updated ones being visible, and
when new hypervisors are added, it will be clear whether they use the
legacy format or not.

Out of tree hypervisors will be broken by this, but the fix is
trivial::

def legacy_nwinfo(self):
    # XXX TODO Update to use the non-legacy format.
    return True

Change-Id: If5b461bc5d8e8dc21de3ca9cf521e7b341724900