virtio: add legacy feature table support

virtio-blk has some legacy feature bits that modern drivers
must not negotiate, but are needed for old legacy hosts
(that e.g. don't support virtio-scsi).
Allow a separate legacy feature table for such cases.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>

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

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