xenserver: Fix vNetManager internal network compatibility with XS 5.5.0.

xenserver: Fix vNetManager internal network compatibility with XS 5.5.0.

The previous commit depends on XAPI to add "xs-network-uuids" external IDs
for internal networks.  Only recent XAPI builds do this and in particular
the version in XenServer 5.5.0 does not.  This commit restores support
for these older XenServer versions.

The xs-network-names field is no longer set.  We plan to avoid the need for
it in the software that once used it.  It was always a bit of a kluge
anyhow.

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