Add a new message, VHOST_USER_SET_VRING_ENABLE, to enable or disable
a specific virt queue, which is similar to attach/detach queue for
tap device.

virtio driver on guest doesn't have to use max virt queue pair, it
could enable any number of virt queue ranging from 1 to max virt
queue pair.
Signed-off-by: NChangchun Ouyang <changchun.ouyang@intel.com>
Signed-off-by: NYuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NYuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: NJason Wang <jasowang@redhat.com>
Tested-by: NMarcel Apfelbaum <marcel@redhat.com>