When the <gic/> element in not present in the domain XML, use the
domain capabilities to figure out what GIC version is usable and
choose that one automatically.

This allows guests to be created on hardware that only supports
GIC v3 without having to update virt-manager and similar tools.

Keep using the default GIC version if the <gic/> element has been
added to the domain XML but no version has been specified, as not
to break existing guests.