conf: introduce hardware UUID (hwuuid) element

conf: introduce hardware UUID (hwuuid) element

The hardware UUID (hwuuid) element provides a mechanism to supply an external
UUID to the guest, as opposed to the libvirt domain UUID. This is to allow
for the scenario whereby a domain can be stopped, cloned and then started as
a new domain without altering the guest-visible UUID.

Add the element, documentation and core code for the hwuuid feature along
with an implementation for the QEMU driver.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>