gconfig: Don't try to return value from gvir_config_domain_set_memory
gconfig: Don't try to return value from gvir_config_domain_set_memory

gvir_config_domain_set_memory() is a void function, so using
g_return_val_if_fail() is not correct. This causes a warning/compilation
failure on CentOS CI framework:
https://ci.centos.org/view/libvirt-project/job/libvirt-glib-build/systems=libvirt-fedora-20/144/console

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