vswitchd: Make names of Bridge external_ids generic.

vswitchd: Make names of Bridge external_ids generic.

Until now the names of the external_ids keys used for Bridge 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.

This finishes the renaming process started in commit c0f9490 "vswitchd:
Make names of Interface external_ids generic."

CC: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>