hw/isa/piix3: Rename "pic" attribute to "isa_irqs_in"

hw/isa/piix3: Rename "pic" attribute to "isa_irqs_in"

TYPE_PIIX3_DEVICE doesn't instantiate a PIC since it relies on the board to do
so. The "pic" attribute, however, suggests that there is one. Rename the
attribute to reflect that it represents ISA interrupt lines. Use the same naming
convention as in the VIA south bridges as well as in TYPE_I82378.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20231007123843.127151-8-shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>