qemu: fix memory leak while starting a guest
    
    In commit cc41c648 I've re-factored qemuMonitorFindBalloonObjectPath, but
    missed that there is a memory leak. The "nextpath" variable is
    overwritten while looping in for cycle and we have to free it before next
    cycle.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>