virtio: use size-based config accessors.

This lets the transport do endian conversion if necessary, and insulates
the drivers from the difference.

Most drivers can use the simple helpers virtio_cread() and virtio_cwrite().

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --cc drivers/block/virtio_blk.c
Simple merge

diff --cc drivers/char/virtio_console.c
Simple merge

diff --cc drivers/net/caif/caif_virtio.c
Simple merge

diff --cc drivers/net/virtio_net.c
Simple merge

diff --cc drivers/scsi/virtio_scsi.c
Simple merge

diff --cc drivers/virtio/virtio_balloon.c
Simple merge

diff --cc net/9p/trans_virtio.c
Simple merge