net/virtio-user: fix feature negotiation

net/virtio-user: fix feature negotiation

The feature negotiation in virtio-user is proven to be broken,
which results in device initialization failure.

Originally, we get features from vhost backend, and remove those
that are not supported. But when new feature is added, for example,
VIRTIO_NET_F_MTU, we fail to remove this new feature. Then, this
new feature will be negotiated, as both frontend and backend claim
to support this feature.

To fix it, we add a macro to record supported features, as a filter
to remove newly added features.

Fixes: 37a7eb2ae816 ("net/virtio-user: add device emulation layer")

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>