Previously we would silently suppress VHOST_USER_PROTOCOL_F_CONFIG
during the protocol negotiation if the QEMU stub hadn't implemented
the vhost_dev_config_notifier. However this isn't the only way we can
handle config messages, the existing vdc->get/set_config can do this
as well.

Lightly re-factor the code to check for both potential methods and
instead of silently squashing the feature error out. It is unlikely
that a vhost-user backend expecting to handle CONFIG messages will
behave correctly if they never get sent.

Fixes: 1c3e5a26 ("vhost-user: back SET/GET_CONFIG requests with a protocol feature")
Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20220321153037.3622127-13-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>