mediacombo: Clear text entry when New VM is launched

We were leaving the ISO field populated with whatever the old value
was. This is likely useful in some cases but it's consistent with
how we handle fields in the rest of the wizard, and has some weird
interaction with OS detection

Fixes: #159

Signed-off-by: Cole Robinson <crobinso@redhat.com>

virtManager/device/mediacombo.py

@@ -153,6 +153,7 @@ class vmmMediaCombo(vmmGObjectUI):
         if not self._rows_inited:
             self._init_rows()
 
+        self._entry.set_text("")
         model = self._combo.get_model()
         model.clear()