hw/virtio: fix Link Control Register for PCI Express virtio devices

Make several Link Control Register flags writable to conform
with the PCI Express spec.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>