lxc: Don't pass a local variable address randomly

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local variable) which is no longer valid once function is finished and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>