gconfig: Fix gvir_config_domain_graphics_new_from_tree
gconfig: Fix gvir_config_domain_graphics_new_from_tree

It does not handle the recently added
GVirConfigDomainGraphicsRdp and GVirConfigDomainGraphicsDesktop
classes.

libvirt/libvirt-glib,libvirt/libvirt-glib,libvirt/libvirt-glib