linux-imx

9p/trans_virtio.c: use virtio_add_sgs[]

virtio_add_buf() is going away, replaced with virtio_add_sgs() which
takes multiple terminated scatterlists.

Cc: Eric Van Hensbergen <ericvh@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>