xenserver: Gracefully refresh network UUIDs on pool join or leave.

xenserver: Gracefully refresh network UUIDs on pool join or leave.

The vswitch database is supposed to maintain an up-to-date UUID for the
system's networks in the Bridge table as external-ids:network-uuids.  On
XenServer systems, /opt/xensource/libexec/interface-reconfigure updates
these fields as bridges are brought up and down.  Most of the time, that is
sufficient.  However, this is one exception: when a XenServer host enters
or leaves a pool, interface-reconfigure is not invoked, and neither is any
other script.  So this commit introduces a new, XenServer-specific daemon
that monitors the XenServer's pool membership status and refreshes the
network UUIDs (by invoking the refresh-network-uuids script) if it changes.

Bug #2097.