hw/virtio/virtio-pmem: Fix definition of VirtIOPMEMClass

hw/virtio/virtio-pmem: Fix definition of VirtIOPMEMClass

VirtIOPMEMClass's parent is VirtioDeviceClass rather than VirtIODevice.
This isn't catastrophic only because sizeof(VirtIODevice) >
sizeof(VirtioDeviceClass).

Fixes: 5f503cd9f388 ("virtio-pmem: add virtio device")
Closes: https://lists.gnu.org/archive/html/qemu-devel/2025-06/msg00586.html
Reported-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Message-ID: <20250606092406.229833-3-zhenzhong.duan@intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>