kvm,virtio: add scatter-gather support v2

The size of the virtqueue(16) was smaller than scatter-gather(128)
which hypervisor tells the guest.  Under some circumstances,
if the guest uses scatter-gather which larger than 16 - 2, kernel panics.
That is why the scatter-gather support v1 breaks Cyrill's kernel boot.

Two descriptors are used as the header and status descriptors.
The remaining descriptors can be used as the real disk data descriptors.
So DISK_SEG_MAX should be VIRTIO_BLK_QUEUE_SIZE - 2.
VIRTIO_BLK_QUEUE_SIZE is 128 and DISK_SEG_MAX is 126 in this patch.

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