docs: clarify multiqueue vs multiple virtqueues

docs: clarify multiqueue vs multiple virtqueues

The vhost-user specification does not explain when
VHOST_USER_PROTOCOL_F_MQ must be implemented.  This may lead
implementors of vhost-user masters to believe that this protocol feature
is required for any device that has multiple virtqueues.  That would be
a mistake since existing vhost-user slaves offer multiple virtqueues but
do not advertise VHOST_USER_PROTOCOL_F_MQ.

For example, a vhost-net device with one rx/tx queue pair is not
multiqueue.  The slave does not need to advertise
VHOST_USER_PROTOCOL_F_MQ.  Therefore the master must assume it has these
virtqueues and cannot rely on askingt the slave how many virtqueues
exist.

Extend the specification to explain the different between true
multiqueue and regular devices with a fixed virtqueue layout.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20190624091304.666-1-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>