[libvirt] [PATCH] leaseshelper: add enhancements to support all events

[Nehal J Wani]

> My concerns are whether this will work in the case you already used the
> leases helper as the patch is adding a few fields to the stored format.

In this patch, I am only adding server-duid as an extra JSON object. I
am not adding more fields to an existing JSON object. The old leases
file will certainly work, even if server-duid is not present.
Server-DUID will get added in any event which will get triggered from
the ipv6 interface. But all of this will happen assuming dnsmasq and
libvirt have been restarted.