drivers/virtio: Clarify CONFIG_VIRTIO_MEM for unsupported architectures

Let's make it clearer that simply unlocking CONFIG_VIRTIO_MEM on an
architecture is most probably not sufficient to have it working as
expected.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Gavin Shan <gshan@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20220610094737.65254-1-david@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>