vhost-user: fix multiple queue specification

The number of queues supported by the slave is queried with
message VHOST_USER_GET_QUEUE_NUM, not with message
VHOST_USER_GET_PROTOCOL_FEATURES.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>