hw/ide/piix: Move registration of VMStateDescription to DeviceClass

hw/ide/piix: Move registration of VMStateDescription to DeviceClass

The modern, declarative way to set up VM state handling is to assign to
DeviceClass::vmsd attribute.

There shouldn't be any change in behavior since dc->vmsd causes
vmstate_register_with_alias_id() to be called on the instance during
the instance init phase. vmstate_register() was also called during the
instance init phase which forwards to vmstate_register_with_alias_id()
internally. Checking the migration schema before and after this patch confirms:

before:
> qemu-system-x86_64 -S
> qemu > migrate -d exec:cat>before.mig

after:
> qemu-system-x86_64 -S
> qemu > migrate -d exec:cat>after.mig

> analyze-migration.py -d desc -f before.mig > before.json
> analyze-migration.py -d desc -f after.mig > after.json
> diff before.json after.json
-> empty

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20230531211043.41724-8-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>