libvhost-user: fix SLAVE_SEND_FD handling

It doesn't look like this could possibly work properly since
VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD is defined to 10, but the
dev->protocol_features has a bitmap. I suppose the peer this
was tested with also supported VHOST_USER_PROTOCOL_F_LOG_SHMFD,
in which case the test would always be false, but nevertheless
the code seems wrong.

Use has_feature() to fix this.

Fixes: d84599f5 ("libvhost-user: support host notifier")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Message-Id: <20190903200422.11693-1-johannes@sipsolutions.net>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>