VIRTIO_BLK_F_CONFIG_WCE removed from virtio 1.0?

I guess we were a bit overzealous in removing VIRTIO_BLK_F_FLUSH aka
VIRTIO_BLK_F_WCE.  The VIRTIO_BLK_F_CONFIG_WCE is a useful feature and
should be preserved in order to achieve feature parity between 0.9 and 1.0.

Sorry for commenting on this so close to the deadline, but I think this
is a blocker. :(  I'll send a patch shortly.

Paolo