virtio: allow per-device-class legacy features

Legacy features are those that transitional devices only
expose on the legacy interface.
Allow different ones per device class.

Cc: qemu-stable@nongnu.org # dependency for the next patch
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
(cherry picked from commit 9b706dbb)

Conflicts:
	hw/virtio/virtio.c

* drop context dep on ff4c07df
* resolv func dep on ff4c07df creating vdc variable in
  virtio_device_class_init()

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>