net/virtio-user: fix advertising of protocol features

net/virtio-user: fix advertising of protocol features

When connected to a vhost-user backend, the flag
VHOST_USER_F_PROTOCOL_FEATURES is not advertised, preventing to do
multiqueue (the VHOST_USER_PROTOCOL_F_MQ protocol feature is ignored by
some backends if the VHOST_USER_F_PROTOCOL_FEATURES feature is not set).

When setting vhost-user features, always advertise this flag.

Fixes: 8e7561054ac7 ("net/virtio: support vhost-user protocol features")
Suggested-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>