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>

docs/interop/vhost-user.txt

@@ -228,8 +228,8 @@ Multiple queue is treated as a protocol extension, hence the slave has to
 implement protocol features first. The multiple queues feature is supported
 only when the protocol feature VHOST_USER_PROTOCOL_F_MQ (bit 0) is set.
 
-The max number of queues the slave supports can be queried with message
-VHOST_USER_GET_PROTOCOL_FEATURES. Master should stop when the number of
+The max number of queue pairs the slave supports can be queried with message
+VHOST_USER_GET_QUEUE_NUM. Master should stop when the number of
 requested queues is bigger than that.
 
 As all queues share one connection, the master uses a unique index for each