lxc: Don't pass a local variable address randomly
    
    So, recently I was testing the LXC driver. You know, startup some
    domains. But to my surprise, I was not able to start a single one:
    
      virsh # start --console test
      error: Reconnected to the hypervisor
      error: Failed to start domain test
      error: internal error: guest failed to start: unexpected exit status 125
    
    So I've start digging. It turns out, that in virExec(), when I printed
    out the @cmd, I got strange values: *(cmd->outfdptr) was certainly not
    valid FD number: it has random value of several millions. This
    obviously made prepareStdFd(childout, STDOUT_FILENO) fail (line 611).
    But outfdptr is set in virCommandSetOutputFD(). The only place within
    LXC driver where the function is called is in
    virLXCProcessBuildControllerCmd(). If you take a closer look at the
    function it looks like this:
    
    static virCommandPtr
    virLXCProcessBuildControllerCmd(virLXCDriverPtr driver,
                                    ..
                                    int logfd,
                                    const char *pidfile)
    {
        ...
        virCommandSetOutputFD(cmd, &logfd);
        virCommandSetErrorFD(cmd, &logfd);
        ...
    }
    
    Yes, you guessed it. @logfd is passed into the function by value.
    However, in the function we try to get its address (an address of a
    local variable) which is no longer valid once function is finished and
    stack is cleaned. Therefore when cmd->outfdptr is evaluated at any
    point after this function, we may get a random number, depending on
    what's currently on the stack. Of course, this may work sometimes too
    - it depends on the compiler how it arranges the code, when the stack
    is wiped out.
    
    In order to fix this, lets pass a pointer to @logfd instead of
    figuring out (wrong) its value in a function.
    
    The bug was introduced in e1de5521.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>