libvirt: per-instance sysinfo serial number

A libvirt guest's serial number in the machine BIOS comes from the https://docs.openstack.org/nova/latest/configuration/config.html#libvirt.sysinfo_serial configuration, which defaults to reading it from the compute hosts' /etc/machine-id file or if that doesn't exist, reading it from the libvirt host capabilities. Either way, all VMs on the same host have the same serial number in the guest BIOS.

Details on sysinfo in libvirt can be found here: https://libvirt.org/formatdomain.html#elementsSysinfo

This blueprint is for adding the ability to have a per-instance serial number, which would likely just be the instance.uuid.

The use case is a VM is running licensed software which reads the serial number from the BIOS. In a disaster recovery situation where the guest is evacuated/rebuilt on another compute host, the serial number within the guest will change which will incur a new licensing fee for the guest running that licensed software. If the guest has a specific serial unique to itself, then the license essentially travels with the guest.