virtio: Formally reserve bits 28-31 to be 'transport' features.

We assign feature bits as required, but it makes sense to reserve some
for the particular transport, rather than the particular device.

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

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

diff --cc include/linux/virtio_config.h
Simple merge