xenserver: Avoid errors from ovs-vsctl at system shutdown.

xenserver: Avoid errors from ovs-vsctl at system shutdown.

Commit 823c5699 "interface-reconfigure: callout to datapath backend class
method on rewrite" changed "interface-reconfigure rewrite" to update
bridge external-ids in the vswitch database.  But this had the side effect
of causing errors at system shutdown, since ovsdb-server gets shut down
before the rewrite action is called.  This commit fixes the problem by
skipping the update if the database socket does not exist.  (It's just
fine to skip the update, since the external-ids will be re-set the next
time the system boots anyhow.)

This commit fixed the problem on 5.6.810-34773p for me.  I don't see the
problem at all on 5.5.0.  Presumably system shutdown order has changed.

NIC-136.

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