gconfig: Fix removal of GVirConfigDomainCpu from domains
gconfig: Fix removal of GVirConfigDomainCpu from domains

A wrong precondition in gvir_config_domain_set_cpu() would reject NULL
GVirConfigDomainCpus instead of accepting them.

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