The vhost-shadow-virtqueue.c build requires include files from
linux-headers/, so it cannot be built on non-Linux systems.
Fortunately it is only needed by vhost-vdpa, so move it there.

Acked-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>