LibvirtBroker: Add & use update_existing_domain

This function will be responsible for updating configuration of
existing/old domains that were created by Boxes. For now it only updates
the "host-passthrough" CPU model using configurations to use custom CPU
model instead (just like we do for new domains now).

LibvirtBroker calls this function on all domains on the default
(session) libvirt connection.

This enables snapshots for existing/old domains that are still using
"host-passthrough".

https://bugzilla.gnome.org/show_bug.cgi?id=735685