libxl: Set def->vcpus after successfully modifying live vcpu count
    
    def->vcpus was never updated after successfully changing the live
    vcpu count of a domain. Subsequent queries for vcpu info would
    return incorrect results.  E.g.:
    
    virsh vcpucount test
    maximum      config         4
    maximum      live           4
    current      config         4
    current      live           4
    
    virsh setvcpus test 2
    
    virsh vcpucount test
    maximum      config         4
    maximum      live           4
    current      config         4
    current      live           4
    
    After patch, live current config is reported correctly:
    
    virsh vcpucount test
    maximum      config         4
    maximum      live           4
    current      config         4
    current      live           2
    
    While fixing this, noticed that the live config was not saved
    to cfg->stateDir via virDomainSaveStatus. Save the live config
    and change error handling of virDomainSave{Config,Status} to
    log a message via VIR_WARN, instead of failing the entire
    DomainSetVcpusFlags operation.
    
    Signed-off-by: Jim Fehlig <jfehlig@suse.com>