qemu: vcpu: Clear vcpu order information rather than making it invalid
    
    Certain operations may make the vcpu order information invalid. Since
    the order is primarily used to ensure migration compatibility and has
    basically no other user benefits, clear the order prior to certain
    operations and document that it may be cleared.
    
    All the operations that would clear the order can still be properly
    executed by defining a new domain configuration rather than using the
    helper APIs.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1370357