Revert "virtio: reject shm region if length is zero"

The commit 206cc44588f7 ("virtio: reject shm region if length is zero")
breaks the Virtio-gpu `host_visible` feature.

As you can see in the snippet below, host_visible_region is zero because
of the `kzalloc`. It's using the `vm_get_shm_region`
(drivers/virtio/virtio_mmio.c:536) to read the `addr` and `len` from
qemu/crosvm.

```
drivers/gpu/drm/virtio/virtgpu_kms.c
132 vgdev = drmm_kzalloc(dev, sizeof(struct virtio_gpu_device), GFP_KERNEL);
[...]
177 if (virtio_get_shm_region(vgdev->vdev, &vgdev->host_visible_region,
178 VIRTIO_GPU_SHM_ID_HOST_VISIBLE)) {
```
Now it always fails.

To fix, revert the offending commit.

Fixes: 206cc44588f7 ("virtio: reject shm region if length is zero")
Signed-off-by: Igor Torrente <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

include/linux/virtio_config.h

···
 bool virtio_get_shm_region(struct virtio_device *vdev,
 			   struct virtio_shm_region *region, u8 id)
 {
-	if (!region->len)
-		return false;
 	if (!vdev->config->get_shm_region)
 		return false;
 	return vdev->config->get_shm_region(vdev, region, id);