ovs-xapi-sync: Handle multiple xs-network-uuids for xs 6.1.

ovs-xapi-sync: Handle multiple xs-network-uuids for xs 6.1.

For xenservers with version less than 6.1, interface reconfiguration
happened through interface-reconfigure scripts in this repo. In cases
where there were multiple xs-network-uuids for a single bridge,
interface-reconfigure script would add the network uuid associated
with the non-VLAN network as the first record. ovs-xapi-sync would
just blindly use the first record to create the bridge-id

But it looks like for xenserver 6.1, interface-reconfigure script
is no longer used and xenserver natively writes the xs-network-uuids.
So, in ovs-xapi-sync we no longer can copy the first value in
xs-network-uuids as bridge-id. This commit fetches the PIF record
for each xs-network-uuids and the network that does not have a VLAN
associated with it is copied over to bridge-id.

Bug #17090.
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>