Add low level APIs to test/set/clear feature bits.
For use by transports, to make it easier to
write code independent of feature bit array format.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/linux/virtio_config.h | 53 ++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 50 insertions(+), 3 deletions(-)

As these functions really set feature bits, I would rather name them:

virtio_has_feature ...
virtio_set_feature ...
virtio_clear_feature ...

But in general, this looks good to me!