Added a new ``unique`` choice to the ``[libvirt]/sysinfo_serial``
configuration which if set will result in the guest serial number
being set to ``instance.uuid`` on this host. It is also made to be
the default value of ``[libvirt]/sysinfo_serial`` config option.
Implements: blueprint per-instance-libvirt-sysinfo-serial
Change-Id: I001beb2840496f7950988acc69018244847aa888