piix4: Rename PIIX4 object to piix4-isa

piix4: Rename PIIX4 object to piix4-isa

Other piix4 parts are already named piix4-ide and piix4-usb-uhci.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Message-Id: <20171216090228.28505-15-hpoussin@reactos.org>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Esteban Bosse <estebanbosse@gmail.com>
[PMD: rebased]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>