IMPORTANT: this is an internal interface consumed only by libvirt. Users should only know about libvirt related impact and new functionality (which is not described here).
Cause: The set_link command is not available in QMP.
Consequence: There's no way to change the network link status through QMP.
Change: Add the set_link command to QMP.
Result: It's now possible to change the network link status through QMP.