qemu: handle -1 for pid in qemuDomainGetMachineName
    
    We call qemuDomainGetMachineName on domain start. On first
    start (after daemon start) pid is 0 and virSystemdGetMachineNameByPID
    don't get called. But after domain shutting down pid became -1 so
    on next start virSystemdGetMachineNameByPID is called and returned an error.
    Error is ignored so it is not critical. But at least on my system
    (systemd-219 with extra patches) systemd-machined is crashed on
    this request.
    
    This behaviour is triggered by eaf2c9f89.
    
    Reviewed-by: Daniel P. Berrange <berrange@redhat.com>