buffer: Change the way the domain localvar is set.

Room ids in newer room versions will become opaque strings and won't
contain the domain of the homeserver.

Get the domain from the homeserver url instead.

matrix/buffer.py

@@ -885,15 +885,11 @@ class RoomBuffer(object):
             buffer_name, server_name, user
         )
 
-        try:
-            _, room_domain = room.room_id.split(":", 1)
         W.buffer_set(
             self.weechat_buffer._ptr,
             "localvar_set_domain",
-                room_domain
+            self.homeserver.hostname
         )
-        except ValueError:
-            pass
 
         W.buffer_set(
             self.weechat_buffer._ptr,