qemu: block: Use correct alias when extracting disk node names
    
    The alias recorded in disk->info.alias is the alias for the frontend
    device but we are interested in the backend drive. This messed up the
    disk node name extraction code as qemu reports the drive alias in the
    block query commands. This was broken in the node name detector
    refactoring done in commit 0175dc6ea024d
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1494327