hw/isa/piix: Allow for optional PIT creation in PIIX3

hw/isa/piix: Allow for optional PIT creation in PIIX3

In the PC machine, the PIT is created in board code to allow it to be
virtualized with various virtualization techniques. So explicitly disable its
creation in the PC machine via a property which defaults to enabled. Once the
PIIX implementations are consolidated this default will keep Malta working
without further ado.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20231007123843.127151-22-shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>