utils: Support composable attributes.

 * pb_type library now has a `get_blif_model` function which works with
   both systems.
 * mux library generates new form of blif_model.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

v2x/lib/mux.py

diff --git a/v2x/lib/mux.py b/v2x/lib/mux.py
index ca31ee2..59db0b2 100644
--- a/v2x/lib/mux.py
+++ b/v2x/lib/mux.py
@@ -140,7 +140,13 @@
     )
 
     if mux_type == MuxType.LOGIC:
-        pb_type_xml.attrib['blif_model'] = '.subckt %s' % subckt
+        add_metadata(pb_type_xml, 'bel', 'mux')
+    else:
+        add_metadata(pb_type_xml, 'bel', 'routing')
+
+    if mux_type == MuxType.LOGIC:
+        model = ET.SubElement(pb_type_xml, "blif_model")
+        model.text = '.subckt {}'.format(subckt)
     else:
         assert not subckt, "Provided subckt={} for non-logic mux!".format(
             subckt