qemuProcessReconnectHelper: Don't create joinable thread
    
    In the qemuProcessReconnectHelper() a new thread that does all the
    interesting work is spawned. The rationale is to not block the daemon
    startup process in case of unresponsive qemu. However, the thread
    handler is a local variable which gets lost once the control goes out of
    scope. Hence the thread gets leaked. We can avoid this if the thread
    isn't made joinable.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>