hw/virtio/virtio-pci: Remove VIRTIO_PCI_FLAG_MIGRATE_EXTRA definition

hw/virtio/virtio-pci: Remove VIRTIO_PCI_FLAG_MIGRATE_EXTRA definition

VIRTIO_PCI_FLAG_MIGRATE_EXTRA was only used by the
hw_compat_2_4[] array, via the 'migrate-extra=true'
property. We removed all machines using that array,
lets remove all the code around VIRTIO_PCI_FLAG_MIGRATE_EXTRA.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Message-ID: <20250512083948.39294-8-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>