vdpa_sim_blk: add support for VIRTIO_BLK_T_FLUSH

The simulator behaves like a ramdisk, so we don't have to do
anything when a VIRTIO_BLK_T_FLUSH request is received, but it
could be useful to test driver behavior.

Let's expose the VIRTIO_BLK_F_FLUSH feature to inform the driver
that we support the flush command.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20220811083632.77525-4-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim_blk.c

@@ -25,6 +25,7 @@
 #define DRV_LICENSE  "GPL v2"
 
 #define VDPASIM_BLK_FEATURES	(VDPASIM_FEATURES | \
+				 (1ULL << VIRTIO_BLK_F_FLUSH)    | \
 				 (1ULL << VIRTIO_BLK_F_SIZE_MAX) | \
 				 (1ULL << VIRTIO_BLK_F_SEG_MAX)  | \
 				 (1ULL << VIRTIO_BLK_F_BLK_SIZE) | \
@@ -188,6 +189,10 @@ static bool vdpasim_blk_handle_req(struct vdpasim *vdpasim,
 		pushed += bytes;
 		break;
 
+	case VIRTIO_BLK_T_FLUSH:
+		/* nothing to do */
+		break;
+
 	default:
 		dev_dbg(&vdpasim->vdpa.dev,
 			"Unsupported request type %d\n", type);