hw/virtio/virtio-mem: Fix definition of VirtIOMEMClass

hw/virtio/virtio-mem: Fix definition of VirtIOMEMClass

Parent of VirtIOMEMClass is VirtioDeviceClass rather than VirtIODevice.
This isn't catastrophic only because sizeof(VirtIODevice) >
sizeof(VirtioDeviceClass).

Fixes: 910b25766b33 ("virtio-mem: Paravirtualized memory hot(un)plug")
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250606092406.229833-2-zhenzhong.duan@intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>