vswitchd: Make names of Interface external_ids generic.

vswitchd: Make names of Interface external_ids generic.

Until now the names of the external_ids keys used for Interface records
have implied that they are specific to XenServer, because they begin with
"xs-".  They are more generic in intent, however, so this commit removes
the "xs-" prefix and explains them more generically.

The Bridge record's external_ids still need renaming.