xenserver: Only put the primary XenServer UUID in default bridge-id

xenserver: Only put the primary XenServer UUID in default bridge-id

This patch defensively guarantees that the first id in
xs-network-uuids will belong to the primary network (as opposed to
a vlan).  Given that the primary network id comes first, it parses
xs-network-ids and only copies the primary id to bridge-id when
monitor-external-ids is run.

Feature #3647

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Reviewed-by: Ben Pfaff <blp@nicira.com>