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: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>