hw/acpi/vmgenid: prevent more than one vmgenid device
A system with multiple VMGENID devices is undefined in the VMGENID spec by
omission.
Cc: "Michael S. Tsirkin" <mst@redhat.com> Cc: Ben Warren <ben@skyportsystems.com> Cc: Igor Mammedov <imammedo@redhat.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Laszlo Ersek <lersek@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Reviewed-by: Igor Mammedov <imammedo@redhat.com>