QOM child properties take a reference to the object and release it when
the property is deleted. Therefore we should unref the default_backend
after we have added it as a child property.
Cc: KONRAD Frederic <fred.konrad@greensocs.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Andreas Färber <afaerber@suse.de>