oslist: Only handle 'activate' if the oslist is visible

Otherwise when we press enter for an already selected OS, it chooses
the first alphabetic entry in the list, overwriting our selection

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

virtManager/oslist.py

@@ -125,6 +125,9 @@ class vmmOSList(vmmGObjectUI):
 
     def _entry_activate_cb(self, src):
         os_list = self.widget("os-list")
+        if not os_list.is_visible():
+            return
+
         sel = os_list.get_selection()
         model, rows = sel.get_selected_rows()
         if rows: