createinterface: raise value error when no ip addr set

We could set blank ip address

IP setting->Configure->Mode (static), then press "OK".

And libvirt will throw an error not very helpful:
Error creating interface: 'Could not define interface:
XML error: could not get interface XML description:
XML invalid - Element protocol failed to validate attributes.

This patch will forbid setting an empty ip address.

Signed-off-by: Chen Hanxiao <chenhanxiao@cn.fujitsu.com>

virtManager/createinterface.py

@@ -764,7 +764,7 @@ class vmmCreateInterface(vmmGObjectUI):
     def build_ip_info(self):
         def build_ip(addr_str):
             if not addr_str:
-                return None, None
+                raise ValueError(_("Please enter an IP address"))
             ret = addr_str.rsplit("/", 1)
             address = ret[0]
             prefix = None