vm-configurator: Add setup_custom_xml()
vm-configurator: Add setup_custom_xml()

Add a public method to setup custom XML on new domains. We'll use this
for domains imported from system libvirt, where most of the config is
mostly copied from an existing domain. Therefore create_domain_config()
is not used and that means update_custom_xml is never called either.

https://bugzilla.gnome.org/show_bug.cgi?id=666185

vbenes/gnome-boxes,vbenes/gnome-boxes