hw/isa/piix3: Modernize reset handling

hw/isa/piix3: Modernize reset handling

Rather than registering the reset handler via a function which
appends the handler to a global list, prefer to implement it as
a virtual method - PIIX4 does the same already.

Note that this means that piix3_reset can now also be called writing to
the relevant configuration space register on a PCI bridge.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20221022150508.26830-6-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>