kvm,virtio: add scatter-gather support

Publish VIRTIO_BLK_F_SEG_MAX feature to guest.

In disk I/O request, guest can use multiple buffers which are not physicall
contiguousy as the I/O request buffer.

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