kvm tools: Bring VIRTIO_BLK_F_SEG_MAX feature back to virtio blk

commit b764422bb0b46b00b896f6d4538ac3d3dde9e56b
(kvm tools: Add support for multiple virtio-blk)
removed the VIRTIO_BLK_F_SEG_MAX publishment to guest.

There is no reason we should not support it. Just bring it back.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>