Howdy,

The following patch improves the -pidfile a fair bit.  -pidfile isn't 
terribly usable at the moment with -daemonize as the wrong pid gets 
written to the file.

Furthermore, there's a race in -pidfile between the stat() and initial 
create.  For a management tool, there's no 100% reliable way to know 
whether there's a stale pidfile or whether another QEMU process is running.

The attached patch uses a common technique for dealing with pidfiles.  
An advisory lock is taken once the pidfile is opened.  The nice thing 
about advisory locks is that the file is unlocked if a process dies.  
This allows for a truly atomic pidfile to be implemented.

AFAIK, lockf() exists on Windows.  I do not know if it works as one 
would expect though.

Regards,

Anthony Liguori